**Steps to replicate the issue** (include links if applicable):
* Open the main page in Firefox (114) in Zebra mode (I use Windows but I doubt that matters; I don't know if this is an issue in not-Zebra mode but in a Firefox private window it can't find the element of interest so I assume it's something that does need to be looked at while logged in).
* Ctrl + U for page source
* Ctrl + F for `vector-sticky-header-search-toggle`
**What happens?**:
Firefox underlines the `button` element with error highlighting as it appears to be slightly malformed: There looks to be a missing space between the `tabindex` and the `data-event-name`.
{F37100341}
The highlighting and missing space is actually repeated several more times below it inside `vector-sticky-header-icons`, for example:
{F37100343}
From what I can tell, the parsing algorithm looks like it handles the missing space just fine as the developer console gets a spaced version, so the error may be something else instead (it's probably the missing space though and Firefox just is smart enough to tell you that other user agents may not render things the way you want due to the lack of space).
{F37100345}
**What should have happened instead?**:
Add a space, at a minimum.
**Software version**: 1.41.0-wmf.12 (e54da81) 15:52, 5 June 2023