Bug 1781102 - Make <hr> styles match the spec. r=TYLin

This matches Chromium, the spec, and fixes the rendering in
https://esta.cbp.dhs.gov/

Spec: https://html.spec.whatwg.org/#the-hr-element-2

Remove -moz-float-edge, to fix the second part of the WPT (also matches
Chromium and the spec which obviously has no such thing).

Differential Revision: https://phabricator.services.mozilla.com/D152670

layout/style/res/html.css

@@ -626,18 +626,21 @@ li {
 /* leafs */
 
 /* <hr> noshade and color attributes are handled completely by
- * the nsHTMLHRElement attribute mapping code
+ * HTMLHRElement::MapAttributesIntoRule.
+ * https://html.spec.whatwg.org/#the-hr-element-2
  */
 hr {
-  display: block;
-  border: 1px inset;
+  color: gray;
+  border-width: 1px;
+  border-style: inset;
   margin-block-start: 0.5em;
   margin-block-end: 0.5em;
   margin-inline-start: auto;
   margin-inline-end: auto;
-  color: gray;
-  -moz-float-edge: margin-box;
-  box-sizing: content-box;
+  overflow: hidden;
+
+  /* FIXME: This is not really per spec */
+  display: block;
 }
 
 hr[size="1"] {

testing/web-platform/meta/html/rendering/non-replaced-elements/the-hr-element-0/hr.html.ini

@@ -1,4 +0,0 @@
-[hr.html]
-  [overflow]
-    expected: FAIL
-

testing/web-platform/meta/html/rendering/non-replaced-elements/the-hr-element-0/setting-overflow-visible.html.ini

@@ -1,7 +0,0 @@
-[setting-overflow-visible.html]
-  [control]
-    expected: FAIL
-
-  [overflow: visible]
-    expected: FAIL
-