[css-color-4] Add 'parse a css color' algorithm, so non-CSS specs using colors don't have to reinvent the machinery here.

Author: tabatkins

css-color-4/Overview.bs

@@ -1115,6 +1115,36 @@ any achromatic color will have a [=powerless=] hue component.
 	still treat the hue component as [=powerless=].
 	It is intentionally unspecified exactly what "sufficiently close" means for this purpose.
 
+<h3 id=parse-color>
+Parsing a <<color>> Value</h3>
+
+<div algorithm>
+	To <dfn export>parse a CSS <<color>> value</dfn>,
+	given a [=string=] |input|,
+	and an optional context <l spec=dom>[=/element=]</l> |element|:
+
+	1. [=CSS/Parse=] |input| as a <<color>>.
+		If the result is failure,
+		return failure;
+		otherwise, let |color| be the result.
+
+	2. If |element| was passed,
+		let |currentcolor| be the [=used value=] of the 'color' property
+		on |element|,
+		and |color-scheme| be the [=used value=] of the 'color-scheme' property
+		on |element|.
+
+		Otherwise,
+		let |currentcolor| and |color-scheme|
+		be the [=initial values=] of 'color' and 'color-scheme', respectively.
+
+	3. Let |used color| be the result of [[#resolving-color-values|resolving]] |color|
+		to a [=used color=],
+		using the |currentcolor| and |color-scheme| values if necessary.
+
+	4. Return |used color|.
+</div>
+