[css-color-4] Add lab to color function, solves #4649

Author: svgeesus

css-color-4/Overview.bs

@@ -2366,7 +2366,19 @@ Predefined colorspaces: ''srgb'', ''display-p3'', ''a98-rgb'', ''prophoto-rgb'',
 
 		<dt><dfn>lab</dfn>
 			<dd>
-				The ''lab'' [[!SRGB]] colorspace accepts three numeric parameters,
+				The ''lab'' [[CIELAB]] colorspace accepts one percentage and two numeric parameters,
+				representing the L, a, and b values.
+				The meaning and valid ranges are <a href="#specifying-lab-lch">the same as the lab() function</a>.
+
+				<div class="example">
+					These are exactly equivalent:
+
+					<pre>
+						<span class="swatch" style="--color: rgb(38.29% 67.27% 93.85%)"></span> lab(67.5345% -8.6911 -41.6019);
+						<span class="swatch" style="--color: rgb(38.29% 67.27% 93.85%)"></span> color(lab 67.5345% -8.6911 -41.6019);
+					</pre>
+				</div>
+
 	</dl>
 
 <h4 id="predefined-to-lab">