Sorry, yes it is html (I spotted the
xml:lang tag and didn't look closer).
Both your link makes clear that: a) That sort of mark-up is a suggested guideline and may not be fully supported. b) That it only applies to MediaWiki.
In the current case adding such mark-up will simply write it out explicitly (since anything else would be unsafe in the xml document where it is used).
The message was updated to support a second parameter (instead of assuming this was gzip). The first parameter is the same as before. The documentation was updated at the same time but I've now also added examples to make things clearer.