Client-side migration for empty li preservation

Client-side migration for empty li preservation

It is desirable in terms of user-friendly syntax to display an empty
list item if the user adds one to the source. However, we suspect that
this change will break the rendering of existing templates. So, preserve
the empty <li> element, but style it with display:none so that there is
no user-visible change. Changes can then be observed with a user script,
then eventually the CSS can be removed so that the desired behaviour will
be user visible.

This is imagined as a staged deployment of T89331, i.e. it is better to
resolve differences with Html5Depurate one at a time instead of
deploying it all at once.

The CSS module is specified in parser/MWTidy.php since the tidy driver
hierarchy is not meant to be so closely tied to the MW environment.

Bug: T49673
Change-Id: Ifb44b782c617240e3de73dcdf76c8737c7307d94