Vanilla discourse supports most (if not everything) of the Markdown standard, a subset of BBCode, and a safe subset of HTML tags.
See here:
https://meta.discourse.org/t/post-format-reference-documentation/19197/2
If you need to know more I would consider diving further into Discourse source code, or by writing a devforum DM to someone who doesn’t mind being spammed and testing a few things there.
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Thanks, I read the post but couldn’t find anything regarding CSS.
Here is a simple test:
Used 'style="color:red;"' inside this h2 tag
If that doesn’t work I guess it doesn’t support standard CSS.