I found the solution.
"Edit As HTML" disappears after switching off Multilingual in
XWiki Preferences.
BTW: I wasn't able to switch multilingual off because of a strange
behaviour:
* set in xwiki.preferences language and def. language to pl,
multilingual to off
* edit Main.WebHome, save
* edit Main.WebHome again - you have still the original (en) content
Appending '?language=pl' to edit url doesn't help.
The same is with left menu.
--
"First they ignore you. Then they laugh at you. Then they
fight you. Then you win." - Mohandas Gandhi.