Follow-up from https://gerrit.wikimedia.org/r/c/mediawiki/core/+/499886
This did affect a few gadgets at the time assuming it would only be inline and thus allow appending things "on the same line".
However, there is a precedent for blocks in the caption, such as div[class=mw-contributions-user-tools] on contributions pages. I've never seen more than one block, though, and that probably look odd design-wise as well if there were multiple lines, so perhaps it makes sense to standardise on <span> for this sort of thing.
Some questions to get started:
- Do the current use cases for .mw-contributions-user-tools and .mw-history-subtitle require the default to be a block (<div>)? It renders the same either way currently (it better, as otherwise adding these wrappers would affect layout and create inconsistencies between those with and without local wrappers).
- Do we generally think it makes sense design-wise for multiple blocks to exist stacked within the subtitle? I suspect not, given tradition is for this line to grow as text. Additional tools must be inserted through dedicated and tailored hooks. See T28934 for an example of that. If there is a need to display unrelated block-level components in this general area, that might be a job for something new or different within a skin or a separate area. Placing it within here is unlikely to work out given the styles applied here generally by skins expect a line of small/muted text.