--- a/layout/style/res/html.css
+++ b/layout/style/res/html.css
@@ -177,67 +177,67 @@ h1 {
   display: block;
   font-size: 2em;
   font-weight: bold;
   margin-block-start: .67em;
   margin-block-end: .67em;
 }
 
 h2,
-:-moz-any(article, aside, nav, section)
+:is(article, aside, nav, section)
 h1 {
   display: block;
   font-size: 1.5em;
   font-weight: bold;
   margin-block-start: .83em;
   margin-block-end: .83em;
 }
 
 h3,
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
 h1 {
   display: block;
   font-size: 1.17em;
   font-weight: bold;
   margin-block-start: 1em;
   margin-block-end: 1em;
 }
 
 h4,
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
 h1 {
   display: block;
   font-size: 1.00em;
   font-weight: bold;
   margin-block-start: 1.33em;
   margin-block-end: 1.33em;
 }
 
 h5,
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
 h1 {
   display: block;
   font-size: 0.83em;
   font-weight: bold;
   margin-block-start: 1.67em;
   margin-block-end: 1.67em;
 }
 
 h6,
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
-:-moz-any(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
+:is(article, aside, nav, section)
 h1 {
   display: block;
   font-size: 0.67em;
   font-weight: bold;
   margin-block-start: 2.33em;
   margin-block-end: 2.33em;
 }
 
@@ -597,36 +597,36 @@ ol {
 }
 
 li {
   display: list-item;
   text-align: match-parent;
 }
 
 /* nested lists have no top/bottom margins */
-:-moz-any(ul, ol, dir, menu, dl) ul,
-:-moz-any(ul, ol, dir, menu, dl) ol,
-:-moz-any(ul, ol, dir, menu, dl) dir,
-:-moz-any(ul, ol, dir, menu, dl) menu,
-:-moz-any(ul, ol, dir, menu, dl) dl {
+:is(ul, ol, dir, menu, dl) ul,
+:is(ul, ol, dir, menu, dl) ol,
+:is(ul, ol, dir, menu, dl) dir,
+:is(ul, ol, dir, menu, dl) menu,
+:is(ul, ol, dir, menu, dl) dl {
   margin-block-start: 0;
   margin-block-end: 0;
 }
 
 /* 2 deep unordered lists use a circle */
-:-moz-any(ol, ul, menu, dir) ul,
-:-moz-any(ol, ul, menu, dir) menu,
-:-moz-any(ol, ul, menu, dir) dir {
+:is(ol, ul, menu, dir) ul,
+:is(ol, ul, menu, dir) menu,
+:is(ol, ul, menu, dir) dir {
   list-style-type: circle;
 }
 
 /* 3 deep (or more) unordered lists use a square */
-:-moz-any(ol, ul, menu, dir) :-moz-any(ol, ul, menu, dir) ul,
-:-moz-any(ol, ul, menu, dir) :-moz-any(ol, ul, menu, dir) menu,
-:-moz-any(ol, ul, menu, dir) :-moz-any(ol, ul, menu, dir) dir {
+:is(ol, ul, menu, dir) :is(ol, ul, menu, dir) ul,
+:is(ol, ul, menu, dir) :is(ol, ul, menu, dir) menu,
+:is(ol, ul, menu, dir) :is(ol, ul, menu, dir) dir {
   list-style-type: square;
 }
 
 
 /* leafs */
 
 /* <hr> noshade and color attributes are handled completely by
  * the nsHTMLHRElement attribute mapping code
@@ -642,32 +642,34 @@ hr {
   -moz-float-edge: margin-box;
   box-sizing: content-box;
 }
 
 hr[size="1"] {
   border-style: solid none none none;
 }
 
-img:-moz-broken::before, input:-moz-broken::before,
-img:-moz-user-disabled::before, input:-moz-user-disabled::before {
+input:is(:-moz-broken, :-moz-user-disabled)::before,
+img:is(:-moz-broken, :-moz-user-disabled)::before {
   content: -moz-alt-content !important;
   unicode-bidi: isolate;
 }
 
-object:-moz-any(:-moz-broken,:-moz-user-disabled) > *|* {
+object:is(:-moz-broken,:-moz-user-disabled) > *|* {
   /*
     Inherit in the object's alignment so that if we aren't aligned explicitly
     we'll end up in the right place vertically.  See bug 36997.  Note that this
     is not !important because we _might_ be aligned explicitly.
   */
   vertical-align: inherit;
 }
 
-img:-moz-suppressed, input:-moz-suppressed, object:-moz-suppressed,
+img:-moz-suppressed,
+input:-moz-suppressed,
+object:-moz-suppressed,
 embed:-moz-suppressed {
   /*
     Set visibility too in case the page changes display.  Note that we _may_
     want to just set visibility and not display, in general, if we find that
     display:none breaks too many layouts.  And if we decide we really do want
     people to be able to right-click blocked images, etc, we need to set
     neither one, and hack the painting code.... :(
    */
@@ -791,25 +793,23 @@ video > .caption-box {
   font: var(--cue-font-size) sans-serif;
   writing-mode: var(--cue-writing-mode, inherit);
   overflow-wrap: break-word;
   /* TODO : enable unicode-bidi, right now enable it would cause incorrect
             display direction, maybe related with bug 1558431. */
 }
 
 /* details & summary */
-details > summary:first-of-type,
-details > summary:-moz-native-anonymous {
+details > summary:is(:first-of-type, :-moz-native-anonymous) {
   display: list-item;
   counter-increment: list-item 0;
   list-style: disclosure-closed inside;
 }
 
-details[open] > summary:first-of-type,
-details[open] > summary:-moz-native-anonymous {
+details[open] > summary:is(:first-of-type, :-moz-native-anonymous) {
   list-style-type: disclosure-open;
 }
 
 details > summary:first-of-type > *|* {
   /* Cancel "list-style-position: inside" inherited from summary. */
   list-style-position: initial;
 }
 
@@ -845,17 +845,17 @@ dialog::backdrop {
 
 marquee {
   inline-size: -moz-available;
   display: inline-block;
   vertical-align: text-bottom;
   text-align: start;
 }
 
-marquee[direction="up"], marquee[direction="down"] {
+marquee:is([direction="up"], [direction="down"]) {
   block-size: 200px;
 }
 
 /* Ruby */
 
 ruby {
   display: ruby;
 }

--- a/layout/style/res/viewsource.css
+++ b/layout/style/res/viewsource.css
@@ -89,13 +89,13 @@ span[id]:before {
 }
 span:not(.error), a:not(.error) {
  unicode-bidi: embed;
 }
 span[id] {
  unicode-bidi: isolate;
 }
 .highlight .error,
-.highlight .error > :-moz-any(.start-tag, .end-tag, .comment, .cdata, .doctype,
-                              .pi, .entity, .attribute-name, .attribute-value) {
+.highlight .error > :is(.start-tag, .end-tag, .comment, .cdata, .doctype,
+                        .pi, .entity, .attribute-name, .attribute-value) {
   color: red;
   font-weight: bold;
 }