… existing filters)

Many existing filters in Core pass segments of the raw HTML markup for matched tags
where code implementing those hooks perform futher analyis on those segments. The
HTML API attempts to hide the raw inner markup as much as possible, making it hard
to provide full backwards compatability with these existing filters.

In this patch a new function is added which extracts the raw markup to maintain
those existing behaviors. It requires a disclaimer to operate to try and encourage
folks to use the structural and semantic methods provided by the HTML API.

Where possible, existing code and filters should be updated so that they no longer
depend on the raw HTML.