[css-color-4] overlapping glyphs and opacity, w3c#5139

Author: svgeesus

css-color-4/Overview.bs

@@ -3077,6 +3077,21 @@ Transparency: the 'opacity' property</h2>
 	an opaque child occluding part of the element's background will continue to do so even when 'opacity' is less than 1,
 	but the element and child as a whole will show the underlying page through themselves.
 
+	<!-- needs a code example and rendered result for those two cases -->
+
+	It also means that the glyphs
+	corresponding to all characters in the element
+	are treated <em>as a whole</em>;
+	any overlapping portions do not increase the opacity.
+
+	<figure>
+		<img src="images/joining-and-transparency.svg"
+		alt="overlapping glphs rendered correctly, and incorrectly">
+		<figcaption>
+			Correct and incorrect rendering of text whose glyphs overlap.
+		</figcaption>
+	</figure>
+
 	If a box has 'opacity' less than 1,
 	it forms a <a>stacking context</a> for its children.
 	(This prevents its contents from interleaving in the z-axis