I want to check some (bigger) sites periodically for changed html pages. There
are some tools (some online), by which I could periodically check a SINGLE
page for changes - but I want to check the whole website. I'm interested in
the changed web pages alone.

Is there a way to do this with httrack? Is there a way to do this without
mirroring the complete sites and keeping their files locally ? Instead, I
could imagine to create hashes from each retrieved page and storing just the
hashes locally.