min-content is standard, so no need to vendor prefix it

Also, makes little sense to say that Firefox sets a `-webkit`-prefixed value

less/forms.less

@@ -11,7 +11,7 @@ fieldset {
   padding: 0;
   margin: 0;
   border: 0;
-  // Chrome and Firefox set a `min-width: -webkit-min-content;` on fieldsets,
+  // 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.
   min-width: 0;