(Actually I've figured this out now, but I'm posting anyway in case it might help someone else with this problem.)
I couldn't get the HTML from online quiz sites to show up. And in my usual stupid way, I couldn't figure out why. So I was writing a bitchy post asking how people do it when I had an idea and tried it. It worked. Classic "rubber dummy theory" in action.
The crucial bit is that the LJ editor doesn't pay proper attention when you change from the "Rich Text" tab to the "HTML" tab. Anything you have already typed is not treated as HTML. You have to change to the HTML tab FIRST and then enter the HTML. If you have already entered it, do a select all, copy, paste, and then try the preview again. Or something like that. It make take a few tries to get it to do it right.
(I suppose this might be a feature instead of a bug, so you can mix both kinds in one post. But it that case there should be something in the FAQ about. I checked, and if it is there it isn't in any place I'd think to look.)
And while we're at it, apparently Livejournal also likes to fuck you over if you have two post windows over at one time, like say, if you are checking how the editor works or something. It flushes the older one. And doesn't save it as a draft.
(And the firefox spelling checker right-click correction still doesn't work.)
(All this is on a WinXP machine with Firefox 126.96.36.199 .)
Are Microsoft and LiveJournal in a bidding war for bad interface writers or something?
Alternatively, I'm just being a stupid FH again.