I support this in theory, but will be a huge undertaking and I'm not sure it's worth the work required for this.
You would have to first create a list of supported code languages, define the syntax for each, run RegEx over the code blocks and add the styling to it.
It makes sense for GitHub to do, but I don't think it makes sense for Glitch.

ah, but this is a solved problem for a very long time: see for example highlight.js

Okay, I played around a bit:
Language detection is wonky at best.
The js test code I used got either detected as arcade or awk.
I would let users specify the language, similar to how github does it.
This needs modification of markdown processing as that is being stripped.
It "works" by making a few adjustments to status_content.js, running highlight on contentHtml.
It's very error prone though, as this needs careful string manipulation to get anything between html tags inside code blocks.

To illustrate the problem:
Specifying js:

Not specifying js (autodetection):

i guess that to be on the safe side, the highlighter should not be run unless the language is explicitly specified – or it should be run with the plaintext format explicitly.

i guess that to be on the safe side, the highlighter should not be run unless the language is explicitly specified – or it should be run with the plaintext format explicitly.

Agreed.
Again, this requires modification to the Markdown processor, because anything immediately after ``` gets stripped, and it needs to be passed to the frontend, so it can be used.
It should not run at all if no language is specified imo though.

Edit:
Ah, it's not the processor, but the sanitation.

I'd really like this, what's the status on the draft PR?

I'd really like this, what's the status on the draft PR?

It should be working fine, but I don't like how much of a mess it became and would rather redo it.
The main issue was that glitch-soc doesn't add a background to code blocks, which messes up highlighting in the reply preview and because of that any language value was available in the preview. The latter was fixed by only allowing valid languages in the backend and remove it there before it's passed to the frontend.
It still needs a change to work with translated toots.

TL;DR: I'm not happy with it in it's current state and needs some updates.