One aspect in which Gerby (the software which underlies the Stacks project) was lacking was its support for TikZ and commutative diagrams. Luckily both The Stacks project and Kerodon are written by people who use xypic for their commutative diagrams, which allows the use of XyJax, but this is not a perfect implementation of xypic.
The system on which (part of) Gerby is built is plasTeX, which contained support for TikZ for a while now, by running pdflatex on every TikZ environment and then converting the resulting pdf to svg. I have now copied over this implementation (thank you Patrick Massot for implementing it for the HTML5 renderer!) and added a minimal working example containing TikZ.
The workflow is exactly the same as before, but I have not tested it outside the minimal working example. One needs to make sure pdf2svg and pdflatex with the appropriate packages are available.