[css-view-transition-2] Test for opt-in to cross-document view transitions does not need to wait for external stylesheets

[martrapp]

From

• External stylesheets in the head are render blocking.
• The pagereveal event fires right before the first frame of the new document is presented.
• The opt in for newDocument is checked when the pagereveal event fires.

I got the impression that you could opt in to view transitions from arbitrary stylesheets linked within the head. In practice, that does not seem to be the case, which is especially annoying because it sometimes works and sometimes does not.

Perhaps the spec could thate more clearly that the browser does not have to wait for @view-transition rules in external stylesheets and you better put the @view-transition rule into a style element within the head, as suggested here: https://issues.chromium.org/issues/383014860#comment9