those criteria mean that any two files could be considered the same, which is a nonsense; which is why I ignored the possibility that you actually meant that; and assumed your description was lacking precision.
No, it is not a nonsense. Imagine me in real life:
1. i'am altering my real name
2. i'am changing the location of where i live
3. i face lift myself.
Am i not still the same person i used to be?
However, inserting a piece of information -- say a custom html-like tag or html comment -- into each html file -- regardless of whether the are 100's 1000's or 100s of 1000s would be the works of a few minutes. At least it would be for Perl.
And what if the .html page gets rewritten by DreamWeaver or Joomla and the certain piece of information is overwwitten, thus gets lost?