Aggregate styles for [hidden] and .hidden

src/scss/common/_setup.scss

@@ -13,10 +13,6 @@ html:is(:root) {
   -webkit-tap-highlight-color: transparent;
 }
 
-[hidden] {
-  display: none !important;
-}
-
 body:where(:not(.user-is-tabbing)) :is(input, select, textarea, button, label, a):focus {
   outline: none;
 }

src/scss/common/_utility.scss

@@ -304,7 +304,7 @@ html:not(.logged-in) body.logged-in .hide-if-logged-in,
 html:not(.logged-in) body:not(.logged-in) .hide-if-logged-out,
 .mobile-menu .hide-in-mobile-menu,
 .site .hide-outside-mobile-menu,
-.hidden,
+.hidden, [hidden],
 .inside-epub {
   display: none !important;
   content-visibility: hidden;