Reboot: Update <fieldset> comment to clarify that the default weirdness is...

Reboot: Update <fieldset> comment to clarify that the default weirdness is part of the standard. (#21261)

Refs https://html.spec.whatwg.org/multipage/#the-fieldset-and-legend-elements
[skip sauce]

scss/_reboot.scss

@@ -347,9 +347,11 @@ textarea {
 }
 
 fieldset {
-  // Chrome and Firefox set a `min-width: min-content;` on fieldsets,
-  // so we reset that to ensure it behaves more like a standard block element.
-  // See https://github.com/twbs/bootstrap/issues/12359.
+  // Browsers set a default `min-width: min-content;` on fieldsets,
+  // unlike e.g. `<div>`s, which have `min-width: 0;` by default.
+  // So we reset that to ensure fieldsets behave more like a standard block element.
+  // See https://github.com/twbs/bootstrap/issues/12359
+  // and https://html.spec.whatwg.org/multipage/#the-fieldset-and-legend-elements
   min-width: 0;
   // Reset the default outline behavior of fieldsets so they don't affect page layout.
   padding: 0;