It looks like we still have to decide which properties are supported before we document them. :)

T162379: Decide which non-standard CSS properties to support in TemplateStyles

I'd be happy to put a list on-wiki if a list exists.

I made a quite mention to Ctrl-F'ing the source for $props in https://www.mediawiki.org/wiki/Help:TemplateStyles#What_CSS_rules_are_recognized?, so folks can at least know how to find a list.

I recently decided to dig in and try to squash this task, originally intending simply to document the allowed properties, but over the course of the work it ended up touching nearly every part of the Help page. The largest part of the new material can be found in the collapsible box at Help:TemplateStyles § Allowed CSS properties and rules on the MediaWiki wiki. Obviously I fully expect that I didn't get everything 100% correct and so I'll gladly revise any mistakes that are spotted; I went into this never having looked at the codebase and possessing an intermediate level (at most) of experience with front-end work. Perhaps with a bit of grace that will have worked to my advantage and helped me find a middle ground between sounding pedantically indifferent or patronizingly insulting. There's still a bit more I want to expand/edit/polish and I'll probably be able to knock that out towards the end of this coming week.

Lastly, I just want to put out there that TemplateStyles is probably my very favorite MW extension in…something like 22 years of having fun working with this software. It unlocked some amazing results that prior to its existence I wouldn't have dared to even hope for and was implemented in such an elegant and mindful way that I can't recall seeing it having been abused even once. My sincere and heartfelt thanks and admiration goes out to everyone that brought it to life and has kept it working smoothly. 🙌