When editing HTML it's easy to make mistakes. Wouldn't it be nice if
there was a simple way to fix these mistakes automatically and tidy up
sloppy editing into nicely layed out markup? Well now there is! Dave
Raggett's HTML TIDY is a free utility for doing just that. It also
works great on the atrociously hard to read markup generated by
specialized HTML editors and conversion tools, and can help you
identify where you need to pay further attention on making your pages
more accessible to people with disabilities.

Tidy is able to fix up a wide range of problems and to bring to your
attention things that you need to work on yourself. Each item found is
listed with the line number and column so that you can see where the
problem lies in your markup. Tidy won't generate a cleaned up version
when there are problems that it can't be sure of how to handle. These
are logged as "errors" rather than "warnings".