It is worth noting that these CSS only numbers/letters will NOT be copied if you select the whole text. It might be problematic. Yet, even without CSS, this is true of the numbers generated by html tags.
But from the accessibility point of view, the text should be the same with or without CSS. This is curious that we have these CSS rules and tricks in the first place, or perhaps is it just an indication that a11y is less equal than other matters...
But for my own point of view, what I'd like to get is an automatic numbering for titles, something that mediawiki does very well, including generating a local TOC. And here, the numbers should be part of the title, albeit in a span of their own to ease CSS tweaking. And this numbering could also give something like 1.3.7.2 or A.3.7.b for an h4 for instance. These title and numbering would perhaps need to be stored in a dictionary. Why? Because, some parts may be relegated to external tiddlers and their numbering be altered during the life of the wiki. But a tiddler needs to keep its title the same as much as can be to avoid problem with linking. For instance if fubar is the tiddler numbered 5.4.7 then in fubar, that numbering could be retrieved from the title name in that dictionary (title->number). A dictionary that could be used in the reverse order would be fine, perhaps a necessity to avoid duplication of information (a great evil!).