tikzjax VS MathJax

Do you want the TeX code to be converted to svg in the browser at rendering time or are you ok with converting the figures to svg beforehand and including them in your html files?

For the former I would look at TikZjaX (https://github.com/kisonecat/tikzjax) which takes tikz code and convert it to svg on the fly, just like MathJaX converting TeX equations. It may be slow, though, especially for complicated plots.

If you are OK with creating the figures beforehand and serving the generated svg, there are several options: Asymptote (https://asymptote.sourceforge.io/) can generate svg and even html files that you can embed in an iframe. You can convert any TeX figure to svg with dvisvgm (play with the --font-format option).

This was my first thought, but the last comment in that page linked above is promising. Someone has the real tikz running in the web browser, rendering to svg: https://github.com/kisonecat/tikzjax It might a simple matter of translating tikz-fenced code blocks to the appropriate script tag and letting that script do its thing.

MathJax - Mathematical Notations expressed using Tex or MathML

Developer tools point to MathJax https://www.mathjax.org/. If you disable javascript you can see some LaTex.

It is something we could add, but it is not planned in the near future. We also have requests for adding math notation (like https://www.mathjax.org/), and that could be a more general solution.

I don't know what Obsidian is, but if it's requiring old TeX math mode toggles (the double dollar sign), then it might not actually be using LaTeX underneath. Many tools that provide LaTeX-style syntax for equations are actually using something like MathJaX, BlahTex, or some custom system by which to translate LaTeX-like syntax into their own equation rendering. This often means you only get a pre-defined subset of what's possible with LaTeX (and the results are never quite faithful to how LaTeX would typeset them).

This will depend heavily on where you're asking the question, e.g. stackexchange has built in mathjax to render it. I'm going to assume you're intending to ask here (because that would make sense), in which case check out the bottom of the sidebar.

From the screenshot, Obsidian looks like a typical Markdown editor that supports some LaTeX math syntax, probably rendered with something like Mathjax. On the other hand, Xournalapp seems to actually use LaTeX, even allowing you to use LaTeX packages like graphicx, tikz, etc.