But in the end went with simple. There will be a very limited subset that will render this way - executing raw code without the users permission in the browser just opens up too much attack surface area that we’d need a very compelling use case to do the security work necessary to make it protected (even if that was possible)
<pre><code>
your mermaid code <span class="foo">with highlighted</span> sections
</code></pre>
Use a tool like pygments to generate the highlighted HTML (assuming there even is highlighting for mermaid syntax).
What you're asking for is an uncommon edgecase that's not what people want in 99% of circumstances (instead they want the rendered chart). Markdown still gives you the HTML escape hatch so use it. Sure it's not pretty but it's no worse than if github didn't support highlighting mermaid source anyways.
If you don't want highlighting just put a fenced code block without the mermaid info string. It will render as unhighlighted code. IMHO that's what you'd want as a workaround to demo source like in a mermaid readme.
> What you're asking for is an uncommon edgecase that's not what people want in 99% of circumstances
It's how it worked before this change, and how it works for every other language in Markdown. This is taking away functionality with no reasonable replacement and creating a special-case gotcha in the Markdown syntax (the same syntax could produce highlighted code or a rendered image depending on the language and what features any particular Markdown implementation supports), and it's not clear why this is necessary when something like ```!mermaid would work just as well to trigger rendering. It's unforced bad language design.
> Use a tool like pygments to generate the highlighted HTML
This requires hardcoding colors into the generated HTML, so the highlighting won't match the color theme of the context that the document is embedded in, breaking e.g. dark mode.
I don't think github should (generally) render stuff in the browser (although I see that both "benign"[1] graphs and source code highlights, not to mention the somewhat simple translation from Markdown to html - probably could be done in the browser as js/wasm.
But for "markdown the language" having a standard syntax for embed/evaluate along with prettify/highlight would've been nice - effectively a standard markup for multi-language "notebooks"/literate programs (maybe we need a triplet: format, render/embed, transclude/literate programming).
Github could then opt-in to highlighting the N languages they do, and initially only try to render mermaid graphs. Then perhaps also graphviz graphs. Etc.
[1] I'm sure one could do some minor DOS at least with a 500 mb graph..
But in the end went with simple. There will be a very limited subset that will render this way - executing raw code without the users permission in the browser just opens up too much attack surface area that we’d need a very compelling use case to do the security work necessary to make it protected (even if that was possible)