This commit implements the fallback to the documents fallback base url
if the href of the first base element is a data or javascript url.
Additionally the frozen base url is set, if a base element becomes the
first base element with an href content attribute because the previous
one got removed.
Previously, we would only invalidate style when setting the `media` IDL
attribute; changing the attribute via `setAttribute()` and
`removeAttribute()` had no immediate effect.
