Just use the spoiler tag in the menu at the top. The only tricky part is the spoiler has to be on its own line - it can't be inline.

You mean hide it under "show entire post" cut?

Thanks Charmingly Euphemistic and Amaranth!

I tried it in Mozilla and it worked! All this time I was wondering what that useless button was for and why can't I edit HTML in HTML pop up window. Now I know - as soon as I re-set Chrome settings, it started working in Chrome, too. I might as well just reinstall the browser, I have other glitches here, come to think of it.
Thanks again!