2D canvas color parsing and serialization

For whatwg/html#8917, w3c/csswg-drafts#10550, and whatwg/html#10481.

Author: annevk

html/canvas/color.window.js

@@ -0,0 +1,64 @@
+const canvas = document.body.appendChild(document.createElement("canvas"));
+const context = canvas.getContext("2d");
+
+[
+  {
+    "input": "rgb(255, 0, 255)",
+    "output": "#ff00ff"
+  },
+  {
+    "input": "rgba(255, 0, 255, 1)",
+    "output": "#ff00ff"
+  },
+  {
+    "input": "#ff00ffff",
+    "output": "#ff00ff"
+  },
+  {
+    "input": "#ff00ff00",
+    "output": "rgba(255, 0, 255, 0)"
+  },
+  {
+    "input": "rgba(255, 0, 255,    0)",
+    "output": "rgba(255, 0, 255, 0)"
+  },
+  {
+    "input": "color(srgb 1 0 1)",
+    "output": "#ff00ff"
+  },
+  {
+    "input": "color(srgb 1 0 1 / 1)",
+    "output": "#ff00ff"
+  },
+  {
+    "input": "color(srgb 1 0 1 / 0)",
+    "output": "rgba(255, 0, 255, 0)"
+  },
+  {
+    "input": "color(srgb 1 none 1)",
+    "output": "color(srgb 1 none 1)" // one could argue for #ff00ff here?
+  },
+  {
+    "input": "color(srgb none -1 2 / 3)", // alpha is clamped to [0, 1] 
+    "output": "color(srgb none -1 2)"
+  },
+  {
+    "input": "color(display-p3 0.96 0.76  \t 0.79)",
+    "output": "color(display-p3 0.96 0.76 0.79)"
+  },
+  {
+    "input": "lab(29% 39 20)",
+    "output": "lab(29 39 20)"
+  },
+  {
+    "input": "transparent",
+    "output": "rgba(0, 0, 0, 0)"
+  }
+].forEach(({ input, output }) => {
+  test(t => {
+    ["fillStyle", "strokeStyle", "shadowColor"].forEach(colorGetterSetter => {
+      context[colorGetterSetter] = input;
+      assert_equals(context[colorGetterSetter], output, colorGetterSetter);
+    });
+  }, `'${input}' should serialize as '${output}'`);
+});