I use server rendered SVG with mathjax as part of my static site generator. To try to make it as accessible as possible I add a title element with an ID to each SVG and use an aria-labelledby attribute to connect the two [1] (a sample for the interested, scroll about half way down [2]). The title content is the unrendered LaTeX source.
I'm very interested in the notion of using HTML and CSS rendering though! Many thanks to the author for pointing out this functionality.