@Tgr Could you please clarify how this task relates to TemplateStyles deployment ruwiki? Thanks!

That's up to the ruwiki community to decide, there never was much concern about duplication on the WMF side. Parsoid-generated HTML is only used in the Android app and in VisualEditor so this should have relatively small impact compared to T187141: Deduplicate template styles in mobileviews API.

VE may not actually want deduplication anyway. If you delete the first transclusion of a template, the styles still need to stay if there are other transclusions. VE could handle this by either not having deduplicated content in the first place or by detecting when a <style data-mw-deduplicate> is being removed and re-duplicate the styles to the first matching <link rel="mw-deduplicated-inline-style"> marker.

Nice to have, but not a blocker for any deployments per T133410#4055687.

Just to confirm, for VE it'd make our lives simpler if this doesn't happen.

In T187142#4521250, @DLynch wrote:

Just to confirm, for VE it'd make our lives simpler if this doesn't happen.

I know, but this is one more instance where reading and editing needs don't always align up perfectly.

We could temporarily delay this, but note that this is not a viable long-term solution since Parsoid HTML will become the default output and at that time, read view performance constraints such as this will take precedence and VE code will have to adapt to that.

That said, this problem is part of a generic issue with extensions that we hope to tackle systematically. (See line 764 of https://gerrit.wikimedia.org/r/#/c/mediawiki/services/parsoid/+/441151/7/lib/ext/Cite/index.js ) .... which is where Parsoid adds an extension-specific handler for dealing with subtrees pre-edit and post-edit. That way, Parsoid deals with global state (that Cite has & TemplateStyles has introduced here for perf reasons) on the server side.

Anyway, https://gerrit.wikimedia.org/r/#/c/mediawiki/services/parsoid/+/452964/ is the parsoid patch to dedupe template styles, but we won't merge that till we chat with you all and see what is needed on the Parsoid end in the short term.

Note that deduplication should only happen after Cite did its own duplication checking of <ref> tags. In the PHP parser that caused T205803: Duplicate reference name errors in English Wikipedia caused by MediaWiki templatestyle handling? (although there the deduplication happens late so the exact cause was different).

In T187142#4667398, @Tgr wrote:

Note that deduplication should only happen after Cite did its own duplication checking of <ref> tags. In the PHP parser that caused T205803: Duplicate reference name errors in English Wikipedia caused by MediaWiki templatestyle handling? (although there the deduplication happens late so the exact cause was different).

Thanks for the heads up! Turns out Parsoid isn't affected by that bug. Only true dupes are flagged since we don't use strip markers in Parsoid.

@Esanders and I discussed this from a VE point of view, and Ed seems to have ideas of how to deal with the deletion case when template styles are deduplicated. We'll go ahead and implement this in Parsoid.

Change 452964 had a related patch set uploaded (by Subramanya Sastry; owner: Subramanya Sastry):
[mediawiki/services/parsoid@master] Dedupe template style occurrences

https://gerrit.wikimedia.org/r/452964

Change 452964 merged by jenkins-bot:
[mediawiki/services/parsoid@master] Dedupe template style occurrences

https://gerrit.wikimedia.org/r/452964

Mentioned in SAL (#wikimedia-operations) [2018-11-28T21:28:30Z] <arlolra> Updated Parsoid to 18a98af (T209236, T210437, T184755, T187142, T208470, T207286, T206777, T205710, T205546, T204477)