>>6365085 Symtom was like browser had a corrupted copy of site's javascript. Already cleared web cache, didn't help. Maybe modem is caching a bad copy. Back later…
Rebooted modem. Didn't make the issue go away.
So assumption is that 8ch/infrastructure is serving up a modified javascript to everybody. I was able to post by using the form at the top of the page (once). After posting, button said "Posted", didn't change back to "New Reply".
So IMO this is sort of hosed up.