[BUGFIX beta] A stack of contextualElements for render buffers, clean omitted start tags

During a renderTree call, keep track of what the contextualElement is and pass this to the buffer reset. This allows `element()` to have the correct context.

Additionally, strip omitted start tags even if the child node causing their generation is after a script tag. See the long comment in `render_buffer`.

Fixes #5476 #5565