Under some very special conditions (namely, performing one particular edit in one particular HTML file), my Emacs session will freeze completely, and starts consuming ~100% of the CPU. The only course of action at this point is to put the poor beast to sleep (with well-aimed kill -9; it won't work without the -9).
I'm trying out characterize the conditions that lead to this behavior.
All I know is that it happens, as I wrote, when I perform a particular edit on this one particular file, AND the major mode for the file is mhtml. (The problem does not occur if I perform the same edit on that same file while the active mode is plain-ol' Fundamental.)
I can't figure out why mhtml is the initial major mode for this file (when I open it).
If I search for mhtml in my .emacs file, I get no hits. Is mhtml the default mode for HTML files?
(For what it's worth, the packages listed by list-packages do not mention mhtml at all.)
Bottom line is: I can't figure out what mhtml mode is, or why I'm getting it by default. It may be something I have in my .emacs, but if so, it's not obvious to me what it is.