Adjust WPT fieldset-generated-content.html to better-target font styling.

Without this change, the test *results* are impossible to read (they're
rendered with the Ahem font) when you run this test directly, because the test
results are stored in nodes that get appended to the body, which means they
inherit the body's "font" declaration.

This avoids that problem by targeting the font CSS to a subtree of test
content (which excludes the test results).

Differential Revision: https://phabricator.services.mozilla.com/D264159

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1987504
gecko-commit: f69c37a305adf3592f7380921c0ff137c7acfce9
gecko-reviewers: layout-reviewers, hiro

Author: dholbert

html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/fieldset-generated-content.html

@@ -4,7 +4,7 @@
 <script src=/resources/testharnessreport.js></script>
 <link rel="stylesheet" href="/fonts/ahem.css">
 <style>
-body {
+.wrapper {
   /* Use Ahem to avoid fractional letter widths. */
   font: 20px/1 Ahem;
 }
@@ -18,8 +18,10 @@
 
 #test::before, #test::after { content:"X"; }
 </style>
-<fieldset id=test><legend>A</legend>Y</fieldset>
-<fieldset id=ref><legend>A</legend>XYX</fieldset>
+<div class="wrapper">
+  <fieldset id=test><legend>A</legend>Y</fieldset>
+  <fieldset id=ref><legend>A</legend>XYX</fieldset>
+</div>
 <script>
 test(() => {
   const testElm = document.querySelector('#test');