Ignore descendant nodes of replace elements during inner text collection steps

[ferjm]

The spec says:

Note that descendant nodes of most replaced elements (e.g., textarea, input, and video — but not button) are not rendered by CSS, strictly speaking, and therefore have no CSS boxes for the purposes of this algorithm.

This makes tests like <audio> contents ignored ("<div><audio>abc") or <video> contents ignored ("<div><video>abc") fail.