diff --git a/blink/renderer/core/css/popup.css b/blink/renderer/core/css/popup.css
index ed0129a0117..eda4ac96c1d 100644
--- a/blink/renderer/core/css/popup.css
+++ b/blink/renderer/core/css/popup.css
@@ -10,13 +10,10 @@
 @namespace "http://www.w3.org/1999/xhtml";
 
 [popup]:-internal-popup-hidden {
-  display: none;
+  display: none !important;
 }
 
-[popup="" i],
-[popup=auto i],
-[popup=hint i],
-[popup=manual i] {
+[popup] {
   position: fixed;
   inset-inline-start: 0;
   inset-inline-end: 0;
@@ -27,20 +24,14 @@
   margin: auto;
   border: solid;
   overflow: auto;
-  padding: 1em;
   color: CanvasText;
   background-color: Canvas;
 }
 
-[popup="" i]::backdrop,
-[popup=auto i]::backdrop,
-[popup=hint i]::backdrop,
-[popup=manual i]::backdrop {
-    position: fixed;
-    inset-inline-start: 0;
-    inset-inline-end: 0;
-    inset-block-start: 0;
-    inset-block-end: 0;
-    background: transparent;
-    pointer-events: none !important;
+[popup]::backdrop {
+  /* From the fullscreen spec: https://fullscreen.spec.whatwg.org/#user-agent-level-style-sheet-defaults: */
+  position: fixed;
+  top:0; right:0; bottom:0; left:0;
+  /* Specific to [popup]: */
+  pointer-events: none !important;
 }
diff --git a/blink/renderer/core/css/selector_checker.cc b/blink/renderer/core/css/selector_checker.cc
index b40284bf422..b113230a524 100644
--- a/blink/renderer/core/css/selector_checker.cc
+++ b/blink/renderer/core/css/selector_checker.cc
@@ -1504,11 +1504,17 @@ bool SelectorChecker::CheckPseudoClass(const SelectorCheckingContext& context,
       }
       return false;
     case CSSSelector::kPseudoPopupHidden:
+      if (!RuntimeEnabledFeatures::HTMLPopupAttributeEnabled(
+              element.GetDocument().GetExecutionContext())) {
+        return false;
+      }
       if (element.HasValidPopupAttribute()) {
         return element.GetPopupData()->visibilityState() ==
                PopupVisibilityState::kHidden;
       }
-      return false;
+      // Invalid values of the `popup` attribute should match the
+      // :-internal-popup-hidden pseudo selector.
+      return element.FastHasAttribute(html_names::kPopupAttr);
     case CSSSelector::kPseudoFullscreen:
     // fall through
     case CSSSelector::kPseudoFullScreen:
diff --git a/blink/web_tests/external/wpt/html/semantics/popups/popup-appearance-ref.tentative.html b/blink/web_tests/external/wpt/html/semantics/popups/popup-appearance-ref.tentative.html
index 95f43dba293..b8ebba794d2 100644
--- a/blink/web_tests/external/wpt/html/semantics/popups/popup-appearance-ref.tentative.html
+++ b/blink/web_tests/external/wpt/html/semantics/popups/popup-appearance-ref.tentative.html
@@ -12,9 +12,8 @@
 </style>
 
 <p>There should be four pop-ups with similar appearance, and
-  the word Unknown with no special styling.</p>
+   the word "Unknown" should not be visible on the page.</p>
 <div class="fake-pop-up" id=blank>Blank</div>
 <div class="fake-pop-up" id=auto>Auto</div>
 <div class="fake-pop-up" id=hint><span>Hint</span></div>
 <div class="fake-pop-up" id=manual>Manual</div>
-<div class="not-a-pop-up!" id=unknown>Unknown</div>
diff --git a/blink/web_tests/external/wpt/html/semantics/popups/popup-appearance.tentative.html b/blink/web_tests/external/wpt/html/semantics/popups/popup-appearance.tentative.html
index 84421317e5a..4acc276c16c 100644
--- a/blink/web_tests/external/wpt/html/semantics/popups/popup-appearance.tentative.html
+++ b/blink/web_tests/external/wpt/html/semantics/popups/popup-appearance.tentative.html
@@ -14,13 +14,13 @@
 </style>
 
 <p>There should be four pop-ups with similar appearance, and
-   the word Unknown with no special styling.</p>
+   the word "Unknown" should not be visible on the page.</p>
 <div popup>Blank
   <div popup=auto>Auto</div>
 </div>
 <div popup=hint>Hint</div>
 <div popup=manual>Manual</div>
-<!-- This ensures unsupported popup values don't receive styling -->
+<!-- This ensures unsupported popup values are hidden -->
 <div popup=unknown>Unknown</div>
 <script>
   document.querySelectorAll('[popup]').forEach(p => {
diff --git a/blink/web_tests/external/wpt/html/semantics/popups/popup-attribute-basic.tentative.html b/blink/web_tests/external/wpt/html/semantics/popups/popup-attribute-basic.tentative.html
index d6090486aa2..1ff7f13c61d 100644
--- a/blink/web_tests/external/wpt/html/semantics/popups/popup-attribute-basic.tentative.html
+++ b/blink/web_tests/external/wpt/html/semantics/popups/popup-attribute-basic.tentative.html
@@ -57,12 +57,14 @@
     parent.appendChild(popUp);
   }
   function assertNotAPopUp(nonPopUp) {
-    // Non-popup elements should already be visible.
-    assert_true(popUpVisible(nonPopUp, /*isPopUp*/false));
+    // If the non-pop-up element nonetheless has a 'popup' attribute, it should
+    // be invisible. Otherwise, it should be visible.
+    const expectVisible = !nonPopUp.hasAttribute('popup');
+    assert_equals(popUpVisible(nonPopUp, /*isPopUp*/false),expectVisible);
     assert_throws_dom("NotSupportedError",() => nonPopUp.showPopUp(),'Calling showPopUp on a non-pop-up should throw NotSupportedError');
-    assert_true(popUpVisible(nonPopUp, /*isPopUp*/false));
+    assert_equals(popUpVisible(nonPopUp, /*isPopUp*/false),expectVisible);
     assert_throws_dom("NotSupportedError",() => nonPopUp.hidePopUp(),'Calling hidePopUp on a non-pop-up should throw NotSupportedError');
-    assert_true(popUpVisible(nonPopUp, /*isPopUp*/false));
+    assert_equals(popUpVisible(nonPopUp, /*isPopUp*/false),expectVisible);
   }
 
   Array.from(document.getElementById('popups').children).forEach(popUp => {
diff --git a/blink/web_tests/external/wpt/html/semantics/popups/popup-hidden-display-ref.tentative.html b/blink/web_tests/external/wpt/html/semantics/popups/popup-hidden-display-ref.tentative.html
index bb321b17ba8..adfcfbf204e 100644
--- a/blink/web_tests/external/wpt/html/semantics/popups/popup-hidden-display-ref.tentative.html
+++ b/blink/web_tests/external/wpt/html/semantics/popups/popup-hidden-display-ref.tentative.html
@@ -2,14 +2,3 @@
 <meta charset="utf-8">
 <link rel=author href="mailto:masonf@chromium.org">
 <link rel="stylesheet" href="resources/popup-styles.css">
-
-<div class=fake-pop-up>This content should be visible</div>
-
-<style>
-  .fake-pop-up {
-    top: 0;
-    left: 0;
-    width: 300px;
-    height: 200px;
-  }
-</style>
diff --git a/blink/web_tests/external/wpt/html/semantics/popups/popup-hidden-display.tentative.html b/blink/web_tests/external/wpt/html/semantics/popups/popup-hidden-display.tentative.html
index 69c82b24827..45c1f91ed43 100644
--- a/blink/web_tests/external/wpt/html/semantics/popups/popup-hidden-display.tentative.html
+++ b/blink/web_tests/external/wpt/html/semantics/popups/popup-hidden-display.tentative.html
@@ -4,11 +4,12 @@
 <link rel=help href="https://open-ui.org/components/popup.research.explainer">
 <link rel=match href="popup-hidden-display-ref.tentative.html">
 
-<div popup>This content should be visible</div>
+<div popup>This content should *not* be visible</div>
+<div popup=invalid>This content should *not* be visible</div>
 
 <style>
   [popup] {
-    display: block; /* This should make the popup visible */
+    display: block !important; /* This should *not* make the popup visible */
     top: 0;
     width: 300px;
     height: 200px;
diff --git a/blink/web_tests/external/wpt/html/semantics/popups/resources/popup-styles.css b/blink/web_tests/external/wpt/html/semantics/popups/resources/popup-styles.css
index 1a99019f060..e1d3e43dccc 100644
--- a/blink/web_tests/external/wpt/html/semantics/popups/resources/popup-styles.css
+++ b/blink/web_tests/external/wpt/html/semantics/popups/resources/popup-styles.css
@@ -9,16 +9,11 @@
   margin: auto;
   border: solid;
   overflow: auto;
-  padding: 1em;
   color: CanvasText;
   background-color: Canvas;
 }
 .fake-pop-up-backdrop {
   position: fixed;
-  inset-inline-start: 0;
-  inset-inline-end: 0;
-  inset-block-start: 0;
-  inset-block-end: 0;
-  background: transparent;
+  top:0; right:0; bottom:0; left:0;
   pointer-events: none !important;
 }