Fixes a bug where we were too eager to remove empty containers.
We already had logic to skip removing empty containers if it had the
filter attribute, which is needed to apply a filter to the whole area.
However, the filter can also be defined through CSS. We did not
properly handle this case, and treated the node as if it had no filter
at all.
This computes the styles and checks the stylesheet as well. (We also
move the logic down to avoid computing the styles eagerly.)
