A few days ago. the Preview function of AWB changed for me (and for at least some other editors, it turns out). It now formats the article in a much more basic way than it always has. For example: the entire page seems to use Times New Roman; broken links are no longer identified in red; the TOC is formatted as a list but not as a box; wikilinks are formatted with underlining. I didn't consciously change any settings. I've tried changing my default browser back to IE, in case Firefox had changed something, but it's made no difference. The page is rendering, but in a pretty rudimentary way, as if it didn't have a full CSS spec to follow.
|Resolved||Rjwilmsi||T143868 Change to formatting in Preview|
|Resolved||Krinkle||T144301 API parse, headhtml properties, key CSS modules such as "mediawiki.action.edit.styles" missing|
It is indeed a CSS issue.
We use an API call to get the mediawiki stylesheet info. That API call reports a lot less stylesheet information than a page preview through browser, hence some formatting is missing.
I have asked on VPT at https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#API_parse.2C_headhtml_properties.2C_modules_such_as_.22mediawiki.action.edit.styles.22_not_returned.3F if API behaviour is as expected, as it would seem to me that the API should tell us the same stylesheet info compared to a non-logged in page preview.
I have tested a local workaround to add in the missing stylesheet information, AWB preview then has box around TOC and shows redlinks, but given different language wikis and other sites like commons etc. may have different setup I very much doubt my workaround will be suitable across the board.