Per-tag CSS files (a power-user feature)
This feature would entail the ability for the user to specify CSS that applies to all HTML (or roundabout HTML, such as MD) files with a specific tag.
For example, I would want the MD files for the documentation of the videogame I'm working on to match a particular color + font scheme.
For all documents tagged Code, as in code samples, I would want white text on a black background.
In order to get the most practical benefit out of this feature, an additional feature would be necessary: In the HTML WYSIWYG, the Styles dropdown would pull in any top level classes from all associated CSS files.
This way, I could have a CSS file for the tag Meeting with classes for people (.person-john, .person-camille) and for products or projects (.pipeline-development, .artist-management). This would allow extremely easy visual categorization of things, so that reviewing meeting notes is easier for others. What's significantly powerful about this approach is that it takes the manual labor out of having a consistent system. If for some reason you change the color coding for the people, that change is instantly reflected across all documents!
This can be done expecting people to remain within the TagSpaces interface to reap this benefit, so that the CSS doesn't need to be pushed around into HTML documents (which would be a major hassle for you guys to code, and potentially a performance problem on lightweight platforms).
This is somewhat similar to OneNote's "tag" feature (which tags individual lines rather than files or documents and is rather limited, but allows for an icon and custom font formatting upon application of the "tag"). The main problem I had with OneNote's implementation is that I had different linetags for different types of documents, but they were stored and presented universally. It was annoying to have GTD-related linetags at the top of the dropdown when I wanted to apply linetags to a meeting record, or vice versa.
Although I've mostly talked about meeting notes, this could also help with medical records, documentation, and much more.
Another use case nicety that this would allow is easy addition, like in OneNote, of little icon/tokens into document margins to indicate the topic of the paragraph they are next to. This is something that a user would need to know complex CSS to pull off (using a :before pseudo-selector, some positioning code, a background-image) and would require the user to have put image files for those icons into a relatively accessible folder. This is a very niche use case, but it exhibits how incredibly extensible this rather simple addition can become given the user's imagination (and isn't too much of a stretch, given this is a favorite feature of the millions of OneNote users). As a side note, this very specific niche use case would be made even more convenient with my suggestion here: https://tagspaces.uservoice.com/forums/213931-general/suggestions/6112104-exclude-specific-tags-by-default
Indeed, it would be extremely useful if TagSpaces supported HTML files in a folder to reference their own css files for styles.