re. 09:00GMT comment --- I think you may have looked at the wrong PR. DokuCssFile->load() is part of PR#407 fixing
FS#2875.
There is only one file changed in the branch,
FS#2879,
https://github.com/splitbrain/dokuwiki/pull/421 -
https://github.com/splitbrain/dokuwiki/blob/47f862d1e038979f4d2dd5bb0c3eaaa9d1ee8fee/lib/exe/css.php
The change is solely concerned with updating relative url values in replacements.
re. 10:28GMT comment --- I have wondered about that. I intend to discuss with other developers at next irc meeting.