I'd recommend holding this off until T186965 and T188143 are solved.

Also please clarify whether T187141: Deduplicate template styles in mobileviews API (style deduplication on the mobile site) and/or T187142: Deduplicate template styles in Parsoid (style deduplication in VisualEditor) are blockers.

Yes, all tasks can be added as blockers. The deployment can wait for a more smooth period according to the others experience of using.

Let's proceed with deployment of TemplateStyles to the Russian Wikipedia in a SWAT window next week.

@Deskana we have two blockers, and we are trying to decide (community discussion) how to circumscribe (without any extension tools) TemplateStyles now (related tasks T188287, T187729).
I also care about T186965.

@Iniquity The discussion linked in the description is dead; the last comment was on 3rd March. It seems like there's really not a lot of interest shown here for beta testing TemplateStyles, so I guess I'll look elsewhere instead.

Does that mean "enable after the blockers are fixed" or "enable as it is now"?

In T188198#4108162, @Deskana wrote:

And, yet, in T187729#3985938, others disagree and say that it's only the consensus to enable it for admins and template editors. From what I can see, things are pretty divided, and I have no interest in fighting over it when I can instead focus on enabling TemplateStyles where there is no such division. This is why the task is marked as stalled.

Don’t really understand how you got to that conclusion based on my post. There is a consensus to enable TemplateStyles, there was division about how to go about with it, but for now it was decided that we would restrict it via proposals in T187729 and then, after there would be more proper discussion about style guidance etc., open this to everyone. Everyone recognises that TemplateStyles is a valuable instrument, there were and are, however, differences in opinions over how to go about it, but it shouldn’t concern you as a developer since it is/will be a community decision.

In T188198#4108230, @Iniquity wrote:

@Deskana We took into account this problem. And we will solve it by internal means.

Okay. I don't speak Russian, so I had no way of knowing this unless someone told me. Thanks for letting me know.

Change 425112 had a related patch set uploaded (by Gergő Tisza; owner: Gergő Tisza):
[operations/mediawiki-config@master] Enable TemplateStyles on ruwiki

https://gerrit.wikimedia.org/r/425112

Change 425112 merged by jenkins-bot:
[operations/mediawiki-config@master] Enable TemplateStyles on ruwiki

https://gerrit.wikimedia.org/r/425112

Mentioned in SAL (#wikimedia-operations) [2018-04-10T13:55:31Z] <tgr@tin> Synchronized wmf-config/InitialiseSettings.php: T188198 Enable TemplateStyles on ruwiki (duration: 01m 00s)

@Tgr, sorry that your test was unsuccessful, we have disabled TemplateStyles functionality for non-sysops/engineers.

Cool, it works.

Small feedback: without using tools like Autoprefixer disabling backwards compatibility via vendor prefixes is uncalled.

In T188198#4120478, @stjn wrote:

Small feedback: without using tools like Autoprefixer disabling backwards compatibility via vendor prefixes is uncalled.

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