CSS can load external resources, such as fonts, images, and stylesheets, from external links.
Couldn’t a malicious site use this see packets dropped and the timestamps for correlation attacks by loading of external clearnet or other servers on the site?
Yes, loading external resources could widen the attack vector and possibilities for a timing attack. And the “safest” Tor browser setting disables loading external resources for this reason.
But this type of advanced timing attacks are only done by advanced actors (e.g state actors) and rather theoretical for most Tor users.
FYI: I’m not an expert or official part of the Tor team, you might want to wait on an answer from a more experienced user.