Style marquee using overflow: hidden

This matches Chromium and WebKit. Tests will be worked on in https://bugzilla.mozilla.org/show_bug.cgi?id=306344.

source

@@ -134347,6 +134347,7 @@ details[open] > summary:first-of-type {
 marquee {
   display: inline-block;
   text-align: initial;
+  overflow: hidden !important;
 }</code></pre>
 
   <p>The <code>marquee</code> element, while <span data-x="concept-marquee-on">turned on</span>, is