algorithm for comparing two html files

I am trying to build a diff tool that allows me to compare two HTML files. I
am looking for resources on how to achive this. The main problem is that I do
not want to simply highlight the line of code where the change happened, but
rather the word/text that changed.

Example say the html file contains a table with three cells/one row, and all
that changes between the two HTML files that I want to compare is the value
on the second cell. I need to be able to distinuish that thats what changed,
even if the actual html code was one single line. (basically comparing what
is being rendered displayed by the HTML renderer).

Any ideas suggestions on where I can start looking ?

thanks