For example, in the following abbreviated test HTML:
<span>some text</span>
<script>println("whf")</script>
We would have to craft the expectation file to include the "some text"
segment, usually with some leading whitespace. This is a bit annoying,
and makes it difficult to manually craft expectation files.
So instead of comparing the expectation against the entire DOM inner
text, we now send the inner text of just the <pre> element containing
the test output when we invoke `internals.signalTextTestIsDone`.
As outlined in: https://www.w3.org/TR/selectors-4/#compat
We now do not treat unknown webkit pseudo-elements as invalid at parse
time, and also support serializing these elements.
Fixes: #21959
