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 thanks to Hewlett Packard's Dave Raggett. HTML TIDY is a
free utility for doing just that. It also works great on the
attrociously 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".