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Unless I hear opposing arguments I will be updating the
patch to handle tabs and to change the default prefix to be
class-level either tonight or tommorow night. 

I plan on adding both pre and post processing to handle tabs.

Preprocessing will be controlled by an an optional keyword
argument in the HtmlDiff constructor specifying the number
of spaces per tab stop.  If absent or None is passed, no
preprocessing will occur.  Otherwise the tabs in each line
of text will be expanded to the specified tab stop spacing
(note, I will use the expandtabs() string method but will
convert the resulting expanded spaces back into tabs so they
get handled by post processing).

Post processing will always occur.  Any tabs remaining in
the HTML markup will be substituted with the contents of a
class-level variable (which can be overridden).  By default,
this variable will be set to ' '

These two changes will allow tab expansion to the correct
tab stop without sacrificing the ability to see differences
in files where tabs were converted to spaces or vice versa.
 It also provides a mechanism where by default, tabs are
reasonably represented (or can be easily changed to your
custom representation).