MediaWiki messages sometimes have HTML tags. This is especially true for apihelp messages. These HTML tags are usually unchanged in the translation, and even if the content of the tag is changed, it would be comfortable to have them clickable (especially in RTL).
I just added definition to https://www.mediawiki.org/wiki/Help:Extension:Translate/Glossary
These live in the translatewiki repository along with the project configuration.
Somewhat belatedly, I wanted to express my appreciation for this patch.
I'm now using this functionality regularly as a translator, and I also found a but thanks to it, which I just fixed: https://gerrit.wikimedia.org/r/#/c/338964/ . It's a very small bug, but I'm sure I'll find this functionality helpful again in the future.