Cover an edge-case with typographic pseudo elements (#80)

* Mention typographic pseudo elements

* Simplify, since it's an edge case

Co-authored-by: Noam Rosenthal <noam@webkit.org>

painttiming.bs

@@ -52,6 +52,7 @@ urlPrefix: https://html.spec.whatwg.org/multipage/dom.html
     type: dfn; text: represents; url: #represents;
 urlPrefix: https://drafts.csswg.org/css-pseudo-4
     type: dfn; text: generated content pseudo-element; url: #generated-content;
+    type: dfn; text: typographical pseudo-element; url: #typographic-pseudos;
 urlPrefix: https://www.w3.org/TR/cssom-view
     type: dfn; text: getBoundingClientRect; url: #dom-element-getboundingclientrect;
     type: dfn; text: scrolling area; url: #scrolling-area;
@@ -122,7 +123,10 @@ A [=CSS image=] |img| is a <dfn>contentful image</dfn> when all of the following
 A {{DOMString}} is <dfn>non-empty</dfn> if it contains at least one character excluding [=document white space characters=].
 
 An [=element=] |target| is <dfn export>contentful</dfn> when one or more of the following apply:
-* |target| has a [=text node=] child, representing [=non-empty=] text.
+* |target| has a [=text node=] child, representing [=non-empty=] text, and the node's [=used=] [=opacity=] is greater than zero.
+
+    NOTE: this covers the case where a [=typographical pseudo-element=] would override the opacity of the text node.
+
 * |target| is a [=replaced element=] representing an [=available=] [=image=].
 * |target| has a [=background-image=] which is a [=contentful image=], and its [=used=] [=background-size=] has non-zero width and height values.
 * |target| is a [=canvas=] with its [=context mode=] set to any value other than <code>none</code>.