What are the restrictions regarding that? Which tags are banned? Is PHP code allowed?

Profiles are mostly CSS based with some HTML tags. I can't say for certain about the PHP or Javascript as I haven't tried to code that much on Subeta.

preeeeeetty sure PHP and javascript don't work on profiles. embeds like video and audio will also be stripped out.

you're correct. back when bug was our developer, he cut all of that out to make using the site safer.

i think they were disabled even before then, i don't remember them ever being an option. could be wrong though.