:is is alias for :matches

src/selector/extend/mod.rs

@@ -656,13 +656,14 @@ impl Extender {
                         // become `.foo:not(.bar)`. However, this is a narrow edge case and
                         // supporting it properly would make this code and the code calling it
                         // a lot more complicated, so it's not supported for now.
-                        if inner_pseudo.normalized_name() == "matches" {
+                        let inner_pseudo_normalized = inner_pseudo.normalized_name();
+                        if inner_pseudo_normalized == "matches" || inner_pseudo_normalized == "is" {
                             inner_pseudo.selector.clone().unwrap().components
                         } else {
                             Vec::new()
                         }
                     }
-                    "matches" | "any" | "current" | "nth-child" | "nth-last-child" => {
+                    "matches" | "is" | "any" | "current" | "nth-child" | "nth-last-child" => {
                         // As above, we could theoretically support :not within :matches, but
                         // doing so would require this method and its callers to handle much
                         // more complex cases that likely aren't worth the pain.

src/selector/parse.rs

@@ -36,9 +36,10 @@ impl DevouredWhitespace {
 }
 
 /// Pseudo-class selectors that take unadorned selectors as arguments.
-const SELECTOR_PSEUDO_CLASSES: [&str; 7] = [
+const SELECTOR_PSEUDO_CLASSES: [&str; 8] = [
     "not",
     "matches",
+    "is",
     "current",
     "any",
     "has",

src/selector/simple.rs

@@ -12,7 +12,7 @@ use super::{
     QualifiedName, SelectorList, Specificity,
 };
 
-const SUBSELECTOR_PSEUDOS: [&str; 4] = ["matches", "any", "nth-child", "nth-last-child"];
+const SUBSELECTOR_PSEUDOS: [&str; 5] = ["matches", "is", "any", "nth-child", "nth-last-child"];
 
 const BASE_SPECIFICITY: i32 = 1000;
 
@@ -487,7 +487,7 @@ impl Pseudo {
     ) -> bool {
         debug_assert!(self.selector.is_some());
         match self.normalized_name() {
-            "matches" | "any" => {
+            "matches" | "is" | "any" => {
                 let pseudos = selector_pseudos_named(compound.clone(), &self.name, true);
                 pseudos.iter().any(move |pseudo2| {
                     self.selector

tests/selector-extend.rs

@@ -296,6 +296,21 @@ test!(
     "a {\n  color: selector-extend(\":not(.c)\", \".c\", \".d:matches(.e, .f)\");\n}\n",
     "a {\n  color: :not(.c):not(.d:matches(.e, .f));\n}\n"
 );
+test!(
+    simple_pseudo_idempotent_not_and_is_list,
+    "a {\n  color: selector-extend(\":not(.c)\", \".c\", \":is(.d, .e)\");\n}\n",
+    "a {\n  color: :not(.c):not(.d):not(.e);\n}\n"
+);
+test!(
+    simple_pseudo_idempotent_not_and_is_list_of_complex,
+    "a {\n  color: selector-extend(\":not(.c)\", \".c\", \":is(.d .e, .f .g)\");\n}\n",
+    "a {\n  color: :not(.c):not(.d .e):not(.f .g);\n}\n"
+);
+test!(
+    simple_pseudo_idempotent_not_and_is_in_compound,
+    "a {\n  color: selector-extend(\":not(.c)\", \".c\", \".d:is(.e, .f)\");\n}\n",
+    "a {\n  color: :not(.c):not(.d:is(.e, .f));\n}\n"
+);
 test!(
     simple_pseudo_idempotent_not_and_not_in_extender,
     "a {\n  color: selector-extend(\":not(.c)\", \".c\", \":not(.d)\");\n}\n",

tests/selector-nest.rs

@@ -37,15 +37,25 @@ test!(
     "a {\n  color: e c d.f;\n}\n"
 );
 test!(
-    nest_parent_in_special_pseudo,
+    nest_parent_in_special_pseudo_matches,
     "a {\n  color: selector-nest(\"c\", \":matches(&)\");\n}\n",
     "a {\n  color: :matches(c);\n}\n"
 );
 test!(
-    nest_complex_super_parent_in_special_pseudo,
+    nest_complex_super_parent_in_special_pseudo_matches,
     "a {\n  color: selector-nest(\"c d\", \":matches(&)\");\n}\n",
     "a {\n  color: :matches(c d);\n}\n"
 );
+test!(
+    nest_parent_in_special_pseudo_is,
+    "a {\n  color: selector-nest(\"c\", \":is(&)\");\n}\n",
+    "a {\n  color: :is(c);\n}\n"
+);
+test!(
+    nest_complex_super_parent_in_special_pseudo_is,
+    "a {\n  color: selector-nest(\"c d\", \":is(&)\");\n}\n",
+    "a {\n  color: :is(c d);\n}\n"
+);
 test!(
     nest_multiple_parent,
     "a {\n  color: selector-nest(\"c\", \"&.d &.e\");\n}\n",
@@ -92,10 +102,15 @@ test!(
     "a {\n  color: e c.f, e d.f;\n}\n"
 );
 test!(
-    nest_super_list_parent_inside_pseudo,
+    nest_super_list_parent_inside_pseudo_matches,
     "a {\n  color: selector-nest(\"c, d\", \":matches(&)\");\n}\n",
     "a {\n  color: :matches(c, d);\n}\n"
 );
+test!(
+    nest_super_list_parent_inside_pseudo_is,
+    "a {\n  color: selector-nest(\"c, d\", \":is(&)\");\n}\n",
+    "a {\n  color: :is(c, d);\n}\n"
+);
 test!(
     nest_super_list_multiple_parent,
     "a {\n  color: selector-nest(\"c, d\", \"&.e &.f\");\n}\n",

tests/selector-parse.rs

@@ -46,6 +46,11 @@ test!(
     "a {\n  color: selector-parse(\":matches(b, c)\");\n}\n",
     "a {\n  color: :matches(b, c);\n}\n"
 );
+test!(
+    pseudo_is_with_list_args,
+    "a {\n  color: selector-parse(\":is(b, c)\");\n}\n",
+    "a {\n  color: :is(b, c);\n}\n"
+);
 test!(
     pseudo_element,
     "a {\n  color: selector-parse(\"::c\");\n}\n",

tests/selector-replace.rs

@@ -21,6 +21,11 @@ test!(
     "a {\n  color: selector-replace(\":matches(c)\", \"c\", \"d\");\n}\n",
     "a {\n  color: :matches(d);\n}\n"
 );
+test!(
+    psuedo_is,
+    "a {\n  color: selector-replace(\":is(c)\", \"c\", \"d\");\n}\n",
+    "a {\n  color: :is(d);\n}\n"
+);
 test!(
     psuedo_not,
     "a {\n  color: selector-replace(\":not(c)\", \"c\", \"d\");\n}\n",

tests/selector-unify.rs

@@ -668,3 +668,13 @@ test!(
     "a {\n  color: selector-unify(\":matches(.c)\", \":matches(.d)\");\n}\n",
     "a {\n  color: :matches(.c):matches(.d);\n}\n"
 );
+test!(
+    simple_pseudo_arg_is_same_selector_arg,
+    "a {\n  color: selector-unify(\":is(.c)\", \":is(.c)\");\n}\n",
+    "a {\n  color: :is(.c);\n}\n"
+);
+test!(
+    simple_pseudo_arg_is_different_selector_arg,
+    "a {\n  color: selector-unify(\":is(.c)\", \":is(.d)\");\n}\n",
+    "a {\n  color: :is(.c):is(.d);\n}\n"
+);

tests/selectors.rs

@@ -526,15 +526,21 @@ test!(
     "a:not(c) {\n  x: y;\n}\n"
 );
 test!(
-    psuedo_paren_removes_inner_placeholder,
+    psuedo_paren_removes_inner_placeholder_matches,
     "a:matches(%b, c) {x: y}",
     "a:matches(c) {\n  x: y;\n}\n"
 );
 test!(
-    matches_placeholder_removes_everything,
+    matches_placeholder_removes_everything_matches,
     "a:matches(%b) {x: y}",
     ""
 );
+test!(
+    psuedo_paren_removes_inner_placeholder_is,
+    "a:is(%b, c) {x: y}",
+    "a:is(c) {\n  x: y;\n}\n"
+);
+test!(is_placeholder_removes_everything_is, "a:is(%b) {x: y}", "");
 test!(
     touching_universal_stays_the_same,
     "a* {\n  color: red;\n}\n",