diff --git a/common/PrefixedPostMessage.js b/common/PrefixedPostMessage.js
new file mode 100644
index 000000000000000..674b52877c0d5d2
--- /dev/null
+++ b/common/PrefixedPostMessage.js
@@ -0,0 +1,100 @@
+/**
+ * Supports pseudo-"namespacing" for window-posted messages for a given test
+ * by generating and using a unique prefix that gets wrapped into message
+ * objects. This makes it more feasible to have multiple tests that use
+ * `window.postMessage` in a single test file. Basically, make it possible
+ * for the each test to listen for only the messages that are pertinent to it.
+ *
+ * 'Prefix' not an elegant term to use here but this models itself after
+ * PrefixedLocalStorage.
+ *
+ * PrefixedMessageTest: Instantiate in testharness.js tests to generate
+ *   a new unique-ish prefix that can be used by other test support files
+ * PrefixedMessageResource: Instantiate in supporting test resource
+ *   files to use/share a prefix generated by a test.
+ */
+var PrefixedMessage = function () {
+  this.prefix = '';
+  this.param = 'prefixedMessage'; // Param to use in querystrings
+};
+
+/**
+ * Generate a URL that adds/replaces param with this object's prefix
+ * Use to link to test support files that make use of
+ * PrefixedMessageResource.
+ */
+PrefixedMessage.prototype.url = function (uri) {
+  function updateUrlParameter (uri, key, value) {
+    var i         = uri.indexOf('#');
+    var hash      = (i === -1) ? '' : uri.substr(i);
+    uri           = (i === -1) ? uri : uri.substr(0, i);
+    var re        = new RegExp(`([?&])${key}=.*?(&|$)`, 'i');
+    var separator = uri.indexOf('?') !== -1 ? '&' : '?';
+    uri = (uri.match(re)) ? uri.replace(re, `$1${key}=${value}$2`) :
+      `${uri}${separator}${key}=${value}`;
+    return uri + hash;
+  }
+  return updateUrlParameter(uri, this.param, this.prefix);
+};
+
+/**
+ * Add an eventListener on `message` but only invoke the given callback
+ * for messages whose object contains this object's prefix. Remove the
+ * event listener once the anticipated message has been received.
+ */
+PrefixedMessage.prototype.onMessage = function (fn) {
+  window.addEventListener('message', e => {
+    if (typeof e.data === 'object' && e.data.hasOwnProperty('prefix')) {
+      if (e.data.prefix === this.prefix) {
+        // Only invoke callback when `data` is an object containing
+        // a `prefix` key with this object's prefix value
+        // Note fn is invoked with "unwrapped" data first, then the event `e`
+        // (which contains the full, wrapped e.data should it be needed)
+        fn.call(this, e.data.data, e);
+        window.removeEventListener('message', fn);
+      }
+    }
+  });
+};
+
+/**
+ * Instantiate in a test file (e.g. during `setup`) to create a unique-ish
+ * prefix that can be shared by support files
+ */
+var PrefixedMessageTest = function () {
+  PrefixedMessage.call(this);
+  this.prefix = `${document.location.pathname}-${Math.random()}-${Date.now()}-`;
+};
+PrefixedMessageTest.prototype = Object.create(PrefixedMessage.prototype);
+PrefixedMessageTest.prototype.constructor = PrefixedMessageTest;
+
+/**
+ * Instantiate in a test support script to use a "prefix" generated by a
+ * PrefixedMessageTest in a controlling test file. It will look for
+ * the prefix in a URL param (see also PrefixedMessage#url)
+ */
+var PrefixedMessageResource = function () {
+  PrefixedMessage.call(this);
+  // Check URL querystring for prefix to use
+  var regex = new RegExp(`[?&]${this.param}(=([^&#]*)|&|#|$)`),
+    results = regex.exec(document.location.href);
+  if (results && results[2]) {
+    this.prefix = results[2];
+  }
+};
+PrefixedMessageResource.prototype = Object.create(PrefixedMessage.prototype);
+PrefixedMessageResource.prototype.constructor = PrefixedMessageResource;
+
+/**
+ * This is how a test resource document can "send info" to its
+ * opener context. It will whatever message is being sent (`data`) in
+ * an object that injects the prefix.
+ */
+PrefixedMessageResource.prototype.postToOpener = function (data) {
+  if (window.opener) {
+    window.opener.postMessage({
+      prefix: this.prefix,
+      data: data
+    }, '*');
+  }
+};
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-innerwidth-innerheight.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-innerwidth-innerheight.html
new file mode 100644
index 000000000000000..7f55f1bb1d809a2
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-innerwidth-innerheight.html
@@ -0,0 +1,75 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: negative values for legacy `innerwidth`, `innerheight`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var windowURL = 'resources/message-opener.html';
+var featuresPrefix = `top=0,left=0,`;
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the features tested here were set to invalid
+  // values in later tests.
+  // In cases where the value for `innerwidth` or `innerheight` is
+  // is less than the browser's minimum allowed value for that dimension,
+  // but NOT 0, the value affected will become the browser's minimum allowed value.
+
+  // This should result in a minimally-sized window for later comparison
+  var featureString = `${featuresPrefix}innerwidth=1,innerheight=1`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+
+  // Negative values for `innerwidth` should result in a window with minimum
+  // valid allowed width
+  [ 'innerwidth=-404',
+    'innerwidth=-404.5',
+    'innerwidth=-404e1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}height=405,${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.width, baselineDimensions.width, `"${feature} is negative and should result in a minimally-wide window"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "width=404"`);
+  });
+
+  // Negative values for `innerheight` should result in a window with minimum
+  // valid allowed height
+  [ 'innerheight=-404',
+    'innerheight=-404.5',
+    'innerheight=-404e1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}width=404,${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.height, baselineDimensions.height, `"${feature} is negative and should result in a minimal-height window"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "height=404"`);
+  });
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-screenx-screeny.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-screenx-screeny.html
new file mode 100644
index 000000000000000..09fb2d6b0a5abdc
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-screenx-screeny.html
@@ -0,0 +1,67 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: negative values for legacy `screenx`, `screeny`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var featuresPrefix = `width=401,height=404,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}top=0,left=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+  // Negative values should be interpreted as 0
+  [ 'screeny=-204',
+    'screeny=-204.5',
+    'screeny=-0'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}left=0,${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.top, baselineDimensions.top, `"${feature} is negative and should be set to 0"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "top=204"`);
+  });
+
+  // Negative values should be interpreted as 0
+  [ 'screenx=-204',
+    'screenx=-204.5',
+    'screenx=-0'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}top=0,${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.left, baselineDimensions.left, `"${feature} is negative and should be set to 0"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "left=204"`);
+  });
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-top-left.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-top-left.html
new file mode 100644
index 000000000000000..15b3103db3cc2e4
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-top-left.html
@@ -0,0 +1,68 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: negative values for `top`, `left`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var featuresPrefix = `width=401,height=404,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}top=0,left=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+
+  // Negative values for `top`, `left` should be interpreted as 0
+  [ 'top=-204',
+    'top=-204.5',
+    'top=-0'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}left=0,${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.top, baselineDimensions.top, `"${feature} is negative and should be set to 0"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "top=204"`);
+  });
+
+// Negative values for `top`, `left` should be interpreted as 0
+  [ 'left=-204',
+    'left=-204.5',
+    'left=-0'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}top=0,${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.left, baselineDimensions.left, `"${feature} is negative and should be set to 0"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "left=204"`);
+  });
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-width-height.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-width-height.html
new file mode 100644
index 000000000000000..30b70926f2c7b38
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-negative-width-height.html
@@ -0,0 +1,75 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: negative values for `width`, `height`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var featuresPrefix = `top=0,left=0,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the features tested here were set to invalid
+  // values in later tests.
+  // In cases where the value for `width` or `height` is
+  // is less than the browser's minimum allowed value for that dimension,
+  // but NOT 0, the value affected will become the browser's minimum allowed value.
+
+  // This should result in a minimally-sized window for later comparison
+  var featureString = `${featuresPrefix}width=1,height=1`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+
+  // Negative values for `width` should result in a window with minimum
+  // valid allowed width
+  [ 'width=-404',
+    'width=-404.5',
+    'width=-404e1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}height=405,${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.width, baselineDimensions.width, `"${feature} is negative and should result in a minimally-wide window"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "width=404"`);
+  });
+
+  // Negative values for `height` should result in a window with minimum
+  // valid allowed height
+  [ 'height=-404',
+    'height=-404.5',
+    'height=-404e1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}width=404,${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.height, baselineDimensions.height, `"${feature} is negative and should result in a minimal-height window"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "height=404"`);
+  });
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-height.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-height.html
new file mode 100644
index 000000000000000..08d127e84b1d2cf
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-height.html
@@ -0,0 +1,91 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: non-integer values for feature `height`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var featuresPrefix = `top=0,left=0,width=401,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}height=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+  // The absence of the sizing feature should have the same behavior
+  // as that feature set to 0
+  [ featuresPrefix,
+    'top=0,left=0',
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.width, baselineDimensions.width);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', feature);
+    }, `${feature}: absence of feature "height" should be treated same as "height=0"`);
+  });
+
+  // When code point in first position is not an ASCII digit, "+" or "-",
+  // that's an error and the value becomes 0
+  [ 'height=/404',
+    'height=_404',
+    'height=L404'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.height, baselineDimensions.height, `"${feature} begins with an invalid character and should be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "height=404"`);
+  });
+
+  // Codepoints that are valid ASCII digits should be collected
+  // Non-ASCII digits and subsequent code points are ignored
+  [ 'height=405.5',
+    'height=405.32',
+    'height=405LLl',
+    'height=405^4',
+    'height=405*3',
+    'height=405/5',
+    'height=405  ',
+    'height=405e1',
+    'height=405e-1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.height, 405, `"${featureString} value after first non-digit will be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should set "height=405"`);
+  });
+
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-innerheight.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-innerheight.html
new file mode 100644
index 000000000000000..5ee752caedc836d
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-innerheight.html
@@ -0,0 +1,76 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: non-integer values for legacy feature `innerheight`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var featuresPrefix = `top=0,left=0,width=401,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}height=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+  // When code point in first position is not an ASCII digit, "+" or "-",
+  // that's an error and the value becomes 0
+  [ 'innerheight=/404',
+    'innerheight=_404',
+    'innerheight=L404'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.height, baselineDimensions.height, `"${feature} begins with an invalid character and should be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "height=404"`);
+  });
+
+  // Codepoints that are valid ASCII digits should be collected
+  // Non-ASCII digits and subsequent code points are ignored
+  [ 'innerheight=405.5',
+    'innerheight=405.32',
+    'innerheight=405LLl',
+    'innerheight=405^4',
+    'innerheight=405*3',
+    'innerheight=405/5',
+    'innerheight=405  ',
+    'innerheight=405e1',
+    'innerheight=405e-1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.height, 405, `"${featureString} value after first non-digit will be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should set "height=405"`);
+  });
+
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-innerwidth.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-innerwidth.html
new file mode 100644
index 000000000000000..972beef48abbdef
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-innerwidth.html
@@ -0,0 +1,75 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: non-integer values for legacy feature `innerwidth`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var featuresPrefix = `top=0,left=0,height=401,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}width=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+  // When code point in first position is not an ASCII digit, "+" or "-",
+  // that's an error and the value becomes 0
+  [ 'innerwidth=/404',
+    'innerwidth=_404',
+    'innerwidth=L404'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.width, baselineDimensions.width, `"${feature} begins with an invalid character and should be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "width=404"`);
+  });
+
+  // Codepoints that are valid ASCII digits should be collected
+  // Non-ASCII digits and subsequent code points are ignored
+  [ 'innerwidth=405.5',
+    'innerwidth=405.32',
+    'innerwidth=405LLl',
+    'innerwidth=405^4',
+    'innerwidth=405*3',
+    'innerwidth=405/5',
+    'innerwidth=405  ',
+    'innerwidth=405e1',
+    'innerwidth=405e-1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.width, 405, `"${featureString} value after first non-digit will be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should set "width=405"`);
+  });
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-left.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-left.html
new file mode 100644
index 000000000000000..fbb2a30ee22687f
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-left.html
@@ -0,0 +1,76 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: non-integer values for feature `left`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+
+var featuresPrefix = `width=401,height=204,top=0,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}left=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+  // When code point in first position is not an ASCII digit, "+" or "-",
+  // that's an error and the value becomes 0
+  [ 'left=/104',
+    'left=_104',
+    'left=L104'
+  ].forEach(feature => {
+    async_test(t => {
+      var featureString = `${featuresPrefix}${feature}`;
+      var prefixedMessage = new PrefixedMessageTest();
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.left, baselineDimensions.left, `"${feature} begins with an invalid character and should be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "left=104"`);
+  });
+
+  // Codepoints that are valid ASCII digits should be collected
+  // Non-ASCII digits and subsequent code points are ignored
+  [ 'left=105.5',
+    'left=105.32',
+    'left=105LLl',
+    'left=105^4',
+    'left=105*3',
+    'left=105/5',
+    'left=105  ',
+    'left=105e1',
+    'left=105e-1'
+  ].forEach(feature => {
+    async_test(t => {
+      var featureString = `${featuresPrefix}${feature}`;
+      var prefixedMessage = new PrefixedMessageTest();
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.left, 105, `"${featureString} value after first non-digit will be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should set "left=105"`);
+  });
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-screenx.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-screenx.html
new file mode 100644
index 000000000000000..2aeab9bb796eac8
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-screenx.html
@@ -0,0 +1,75 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: non-integer values for legacy feature `screenx`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var featuresPrefix = `width=401,height=204,top=0,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}left=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+  // When code point in first position is not an ASCII digit, "+" or "-",
+  // that's an error and the value becomes 0
+  [ 'screenx=/104',
+    'screenx=_104',
+    'screenx=L104'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.left, baselineDimensions.left, `"${feature} begins with an invalid character and should be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "left=104"`);
+  });
+
+  // Codepoints that are valid ASCII digits should be collected
+  // Non-ASCII digits and subsequent code points are ignored
+  [ 'screenx=105.5',
+    'screenx=105.32',
+    'screenx=105LLl',
+    'screenx=105^4',
+    'screenx=105*3',
+    'screenx=105/5',
+    'screenx=105  ',
+    'screenx=105e1',
+    'screenx=105e-1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.left, 105, `"${featureString} value after first non-digit will be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should set "left=105"`);
+  });
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-screeny.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-screeny.html
new file mode 100644
index 000000000000000..cb4e873f26d022f
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-screeny.html
@@ -0,0 +1,76 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: non-integer values for legacy feature `screeny`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var featuresPrefix = `height=401,width=402,left=0,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}top=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+  // When code point in first position is not an ASCII digit, "+" or "-",
+  // that's an error and the value becomes 0
+  [ 'screeny=/404',
+    'screeny=_404',
+    'screeny=L404'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.top, baselineDimensions.top, `"${feature} begins with an invalid character and should be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "height=404"`);
+  });
+
+  // Codepoints that are valid ASCII digits should be collected
+  // Non-ASCII digits and subsequent code points are ignored
+  [ 'screeny=405.5',
+    'screeny=405.32',
+    'screeny=405LLl',
+    'screeny=405^4',
+    'screeny=405*3',
+    'screeny=405/5',
+    'screeny=405  ',
+    'screeny=405e1',
+    'screeny=405e-1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.top, 405, `"${featureString} value after first non-digit will be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should set "height=405"`);
+  });
+
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-top.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-top.html
new file mode 100644
index 000000000000000..d4f4e90d96ed638
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-top.html
@@ -0,0 +1,73 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: non-integer values for feature `top`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var windowURL = 'resources/message-opener.html';
+var featuresPrefix = `width=401,height=204,left=0,`;
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}top=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+  // When code point in first position is not an ASCII digit, "+" or "-",
+  // that's an error and the value becomes 0
+  [ 'top=/104',
+    'top=_104',
+    'top=L104'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.top, baselineDimensions.top, `"${feature} begins with an invalid character and should be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "top=104"`);
+  });
+
+  // Codepoints that are valid ASCII digits should be collected
+  // Non-ASCII digits and subsequent code points are ignored
+  [ 'top=105.5',
+    'top=105.32',
+    'top=105LLl',
+    'top=105^4',
+    'top=105*3',
+    'top=105/5',
+    'top=105  ',
+    'top=105e1',
+    'top=105e-1'
+  ].forEach(feature => {
+    async_test(t => {
+      var featureString = `${featuresPrefix}${feature}`;
+      var prefixedMessage = new PrefixedMessageTest();
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.top, 105, `"${feature} value after first non-digit will be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should set "top=105"`);
+  });
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-width.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-width.html
new file mode 100644
index 000000000000000..a746abed3f36abe
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-non-integer-width.html
@@ -0,0 +1,91 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: non-integer values for feature `width`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var featuresPrefix = `top=0,left=0,height=401,`;
+var windowURL = 'resources/message-opener.html';
+
+// https://html.spec.whatwg.org/multipage/infrastructure.html#rules-for-parsing-integers
+
+setup (() => {
+  // Before running tests, open a window using features that mimic
+  // what would happen if the feature tested here were set to 0
+  // for comparison later.
+  var featureString = `${featuresPrefix}width=0`;
+  var prefixedMessage = new PrefixedMessageTest();
+  prefixedMessage.onMessage((data, e) => {
+    e.source.close();
+    runWindowTests(data);
+  });
+  var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+});
+
+function runWindowTests (baselineDimensions) {
+
+  // The absence of the sizing feature should have the same behavior
+  // as that feature set to 0
+  [ featuresPrefix,
+    'top=0,left=0',
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.width, baselineDimensions.width);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', feature);
+    }, `${feature}: absence of feature "width" should be treated same as "width=0"`);
+  });
+
+  // When code point in first position is not an ASCII digit, "+" or "-",
+  // that's an error and the value becomes 0
+  [ 'width=/404',
+    'width=_404',
+    'width=L404'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.width, baselineDimensions.width, `"${feature} begins with an invalid character and should be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should NOT set "width=404"`);
+  });
+
+  // Codepoints that are valid ASCII digits should be collected
+  // Non-ASCII digits and subsequent code points are ignored
+  [ 'width=405.5',
+    'width=405.32',
+    'width=405LLl',
+    'width=405^4',
+    'width=405*3',
+    'width=405/5',
+    'width=405  ',
+    'width=405e1',
+    'width=405e-1'
+  ].forEach(feature => {
+    async_test(t => {
+      var prefixedMessage = new PrefixedMessageTest();
+      var featureString = `${featuresPrefix}${feature}`;
+      prefixedMessage.onMessage(t.step_func_done((data, e) => {
+        e.source.close();
+        assert_equals(data.width, 405, `"${featureString} value after first non-digit will be ignored"`);
+      }));
+      var win = window.open(prefixedMessage.url(windowURL), '', featureString);
+    }, `features "${feature}" should set "width=405"`);
+  });
+}
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-innerheight-innerwidth.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-innerheight-innerwidth.html
new file mode 100644
index 000000000000000..c839c6ca0661efc
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-innerheight-innerwidth.html
@@ -0,0 +1,55 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: tokenization -- legacy size features `innerheight`, `innerwidth`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var windowURL = 'resources/message-opener.html';
+var width = 'width=401,';
+var height = 'height=402,';
+
+[ 'innerwidth=401',
+  ' innerwidth = 401',
+  'innerwidth==401',
+  '\ninnerwidth= 401',
+  ',innerwidth=401,,',
+  'INNERWIDTH=401',
+  'innerWidth=401'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.width, 401);
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', height + features);
+  }, `${format_value(features)} should set width of opened window`);
+});
+
+[ 'innerheight=402',
+  ' innerheight = 402',
+  'innerheight==402',
+  '\ninnerheight= 402',
+  ',innerheight=402,,',
+  'INNERHEIGHT=402',
+  'innerHeight=402'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.height, 402);
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', width + features);
+  }, `${format_value(features)} should set height of opened window`);
+});
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-001.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-noopener.html
similarity index 99%
rename from html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-001.html
rename to html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-noopener.html
index 3d3f53e185f5fc1..55e8b44d5dfbf50 100644
--- a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-001.html
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-noopener.html
@@ -1,6 +1,7 @@
 <!DOCTYPE html>
 <meta charset="utf-8">
 <title>HTML: window.open `features`: tokenization -- `noopener`</title>
+<meta name=timeout content=long>
 <link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-screenx-screeny.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-screenx-screeny.html
new file mode 100644
index 000000000000000..bb6fc4b84bfb615
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-screenx-screeny.html
@@ -0,0 +1,55 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: tokenization -- legacy position features `screenx`, `screeny`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var windowURL = 'resources/message-opener.html';
+var width = 'width=401,';
+var height = 'height=402,';
+
+[ 'screenx=141',
+  ' screenx = 141',
+  'screenx==141',
+  '\nscreenx= 141',
+  ',screenx=141,,',
+  'SCREENX=141',
+  'screenX=141'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.left, 141);
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', width + height + features);
+  }, `${format_value(features)} should set left position of opened window`);
+});
+
+[ 'screeny=142',
+  ' screeny = 142',
+  'screeny==142',
+  '\nscreeny= 142',
+  ',screeny=142,,',
+  'SCREENY=142',
+  'screenY=142'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.top, 142);
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', width + height + features);
+  }, `${format_value(features)} should set top position of opened window`);
+});
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-top-left.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-top-left.html
new file mode 100644
index 000000000000000..ef098a1062c6bc0
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-top-left.html
@@ -0,0 +1,69 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: tokenization -- position features `top` and `left`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var windowURL = 'resources/message-opener.html';
+var width = 'width=401,';
+var height = 'height=402,';
+
+[ 'left=141',
+  ' left = 141',
+  'left==141',
+  '\nleft= 141',
+  ',left=141,,',
+  'LEFT=141'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.left, 141);
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', width + height + features);
+  }, `"${features}" should set left position of opened window`);
+});
+
+[ 'top=142',
+  ' top = 142',
+  'top==142',
+  '\ttop= 142',
+  ',top=142,,',
+  'TOP=142'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.top, 142);
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', width + height + features);
+  }, `${format_value(features)} should set top position of opened window`);
+});
+
+[ 'top=152,left=152',
+  'top=152,,left=152,',
+  'top=152==left=152',
+  ',,top= 152, left=152'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.top, 152);
+      assert_equals(data.left, 152);
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', width + height + features);
+  }, `${format_value(features)} should set top and left position of opened window`);
+});
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-width-height.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-width-height.html
new file mode 100644
index 000000000000000..cac4ae639b2f0e0
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/open-features-tokenization-width-height.html
@@ -0,0 +1,71 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>HTML: window.open `features`: tokenization -- size features `width` and `height`</title>
+<meta name=timeout content=long>
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#apis-for-creating-and-navigating-browsing-contexts-by-name">
+
+<!-- user agents are not required to support open features other than `noopener`
+     and on some platforms position and size features don't make sense -->
+<meta name="flags" content="may" />
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var windowURL = 'resources/message-opener.html';
+var width = 'width=401,';
+var height = 'height=402,';
+
+[ 'width=401',
+  ' width = 401',
+  'width==401',
+  '\nwidth= 401',
+  ',width=401,,',
+  'WIDTH=401'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.width, 401);
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', height + features);
+  }, `${format_value(features)} should set width of opened window`);
+});
+
+[ 'height=402',
+  ' height = 402',
+  'height==402',
+  '\nheight= 402',
+  ',height=402,,',
+  'HEIGHT=402'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.height, 402);
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', width + features);
+  }, `${format_value(features)} should set height of opened window`);
+});
+
+[ 'height=402,width=401',
+  ' height = 402 , width = 401 ,',
+  'height==402 width = 401',
+  '\nheight= 402,,width=\n401',
+  ',height=402,,width==401',
+  'HEIGHT=402, WIDTH=401'
+].forEach((features, idx, arr) => {
+  async_test(t => {
+    var prefixedMessage = new PrefixedMessageTest();
+    prefixedMessage.onMessage(t.step_func_done((data, e) => {
+      e.source.close();
+      assert_equals(data.height, 402);
+      assert_equals(data.width, 401)
+    }));
+    var win = window.open(prefixedMessage.url(windowURL), '', features);
+  }, `${format_value(features)} should set height and width of opened window`);
+});
+
+</script>
diff --git a/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/resources/message-opener.html b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/resources/message-opener.html
new file mode 100644
index 000000000000000..e6c164bfae53f42
--- /dev/null
+++ b/html/browsers/the-window-object/apis-for-creating-and-navigating-browsing-contexts-by-name/resources/message-opener.html
@@ -0,0 +1,12 @@
+<script src="/common/PrefixedPostMessage.js"></script>
+<script>
+var prefixedMessage = new PrefixedMessageResource();
+window.addEventListener('load', () => {
+  prefixedMessage.postToOpener({
+    left: window.screenX,
+    top: window.screenY,
+    width: window.innerWidth,
+    height: window.innerHeight
+  });
+});
+</script>