Merge pull request #561 from upsuper/color-glyph-note

[css-color] Add note for relationship between color property and color glyphs

Author: svgeesus

css-color/Overview.bs

@@ -86,6 +86,15 @@ For example, to specify lime green:
 		<dd>The <<color>> type is defined in a later section.
 	</dl>
 
+    <p class="note">
+    Note: In general, this property, including its alpha component, has
+    no effect on "color glyphs", such as emoji in some fonts, which are
+    colored by a built-in palette. Some colored fonts are able to refer
+    to the foreground color, such as palette entry ''0xFFFF'' in COLR
+    table of OpenType, and ''context-fill'' value in SVG-in-OpenType. In
+    that case, the foreground color is set by this property, identical
+    to how ''<color>/currentcolor'' value works.
+
 <h2 id='color-type'>
 Representing sRGB Colors: the <<color>> type</h2>