The mobile-web tag info
For questions about the mobile versions of Stack Exchange sites. You can access the mobile version of a site by clicking the "mobile" link in the page footer.
is outdated because there are no “mobile versions” of the sites anymore, and there is no “mobile” link in the page footer, see Deprecating our mobile views :
On March 2, 2022 we removed the Mobile button from the footer. This means the mobile views are removed entirely from Stack Overflow.
The mobile-web tag info on the main meta now states
DO NOT USE THIS TAG FOR NEW QUESTIONS UNLESS THEY ARE ABOUT CHAT! Use in combination with [chat] for posts about the mobile versions of the chat sites. The mobile views of the main sites have been removed.
For questions about the site's current design on mobile-sized screens and devices, use responsive-design instead.
I suggest that the mobile-web tag info is updated in a similar spirit, something like
DO NOT USE THIS TAG FOR NEW QUESTIONS. This used to be for questions about the mobile versions of Stack Exchange sites.
For new question about the design of the Mathematics site on mobile-sized screens and devices, use design instead.
Of course most question about design or display problems on specific devices would be posted on the main meta, but there are cases where such problems seem to occur on Mathematics only.