See rETST99640093465f: Cache processed stylesheets during the parse. The TemplateStyles parsing process does not depend on parser options, so we could just cache in some global location instead; that makes it hard to say when the cache is not needed anymore, but I doubt that would cause any practical problems.

TemplateStyles parsing is not actually context-independent (as we want the first occurrence of a <templatestyles> reference to a given CSS page to turn into a <style> tag but any subsequent ones into a no-op reference to that styles tag) and Parsoid will have to accommodate that somehow, but this specific attribute is not related to that.

In T343226#9061363, @Tgr wrote:

TemplateStyles parsing is not actually context-independent (as we want the first occurrence of a <templatestyles> reference to a given CSS page to turn into a <style> tag but any subsequent ones into a no-op reference to that styles tag) and Parsoid will have to accommodate that somehow, but this specific attribute is not related to that.

We handle this with a post processing step at the very end of the parse, when all the fragments have been assembled, which deduplicates the output. This is already present in ParserOutput::getText() I believe (option deduplicateStyles).

Oh right, sorry, I forgot we ended up implementing that outside the parser.

Change 1013134 had a related patch set uploaded (by Umherirrender; author: Umherirrender):

[mediawiki/extensions/TemplateStyles@master] Create TemplateStylesContentProvider service to hold and fill cache

https://gerrit.wikimedia.org/r/1013134