undo got turned off

[Dan Jacobson]

Gentlemen, how cruel of you to beep "undo-start: No undo information
in this buffer". One never knew how much one uses undo until it is
turned off. All one can do is save to a new file for the moment.

Save the attached into file.html
then do emacs -Q file.html
type a character, try to undo, and get a beep: undo has been turned off!
(Also it is in sgml mode, not html(-helper) mode.)

Probably the mixed charsets caused it. OK,
  $ validate file.html
  *** Errors validating file.html: ***
  Error at line 6, character 164:  non SGML character number 137
  Error at line 6, character 168:  non SGML character number 138
But why turn off undo in revenge?

Got an idea: how about a question in the minibuffer:
   file.html is not in a familiar charset! Proceed? (y,n,q,?...)

http://radioscanningtw.jidanni.org/index.php?title=特殊:Recentchanges&hidemyself=1
http://www.google.com.tw/search?hl=zh-TW&ie=Big5&safe=off&q=FM56R-NFV2 linux&btnG=·j´M&meta=