PS: I just accessed the SMF forum via the link you provided, from my Firefox browser on ArcaOs 5.1.2 and it loads and looks great. I didn't observe any browser issues or errors.
Likewise, it seemed to display fine on SeaMonkey, both with and without JavaScript enabled. No account so couldn't try posting.
Hmm, tried registering and keep getting this message,
The following errors were detected in your registration. Please correct them to continue:
The letters you typed do not match the letters that were shown in the picture.
Don't see a picture or captcha in SM.
So tried registering with Dooble-Qt6, lots of clicking images for the captcha, then the system froze.
Rebooted and tried with Dooble-Qt5, lots of clicking images and eventually was registered
Trying to post fails on SM, no visible capta and trying to post in Dooble-Qt5 does the captcha thing and afterwards won't let me post with another error about letters not matching picture.
So not much luck actually posting, or previewing which is what I tried.