Use css-color module

benchmark/dom/get-computed-style.js

@@ -0,0 +1,48 @@
+"use strict";
+const suite = require("../document-suite");
+
+exports["getComputedStyle() on document.body"] = () => {
+  let window, document;
+
+  return suite({
+    setup(doc) {
+      window = doc.defaultView;
+      document = doc;
+    },
+    fn() {
+      window.getComputedStyle(document.body);
+    }
+  });
+};
+
+exports["getComputedStyle() on element with rgb color"] = () => {
+  let window, node;
+
+  return suite({
+    setup(doc) {
+      window = doc.defaultView;
+      node = doc.createElement("div");
+      node.style.color = "rgb(128, 0, 128)";
+      doc.body.appendChild(node);
+    },
+    fn() {
+      window.getComputedStyle(node);
+    }
+  });
+};
+
+exports["getComputedStyle() on element with non-rgb color"] = () => {
+  let window, node;
+
+  return suite({
+    setup(doc) {
+      window = doc.defaultView;
+      node = doc.createElement("div");
+      node.style.color = "color-mix(in srgb, red, blue)";
+      doc.body.appendChild(node);
+    },
+    fn() {
+      window.getComputedStyle(node);
+    }
+  });
+};

benchmark/dom/index.js

@@ -5,5 +5,6 @@ module.exports = {
   "tree-modification": require("./tree-modification"),
   "compare-document-position": require("./compare-document-position"),
   "named-properties": require("./named-properties"),
-  "inner-html": require("./inner-html")
+  "inner-html": require("./inner-html"),
+  "get-computed-style": require("./get-computed-style")
 };

lib/jsdom/living/helpers/colors.js

@@ -1,5 +1,7 @@
 "use strict";
 
+const { resolve: getResolvedValueForColor } = require("@asamuzakjp/css-color");
+
 // https://drafts.csswg.org/css-color-4/#named-color
 const namedColors = {
   __proto__: null,
@@ -193,11 +195,11 @@ function sharedSpecifiedAndComputedAndUsed(color) {
     return hexToRGBA(color.slice(1));
   }
 
-  if (/^rgba?\(/.test(color)) {
+  if (/^rgba?\(/.test(color) && color.includes(",")) {
     return color.split(",").map(s => s.trim()).join(", ");
   }
 
-  return color;
+  return getResolvedValueForColor(color);
 }
 
 function hexToRGB(color) {

package.json

@@ -20,6 +20,7 @@
   "license": "MIT",
   "repository": "jsdom/jsdom",
   "dependencies": {
+    "@asamuzakjp/css-color": "^1.0.1",
     "cssstyle": "^4.0.1",
     "data-urls": "^5.0.0",
     "decimal.js": "^10.4.3",