When working on a long running projects, it’s easy to lose track of HTML and CSS standard compliance. Also, link rot is a common occurrence. Gladly, there are command line tools that can be executed on a regular basis (think weekly or monthly cron jobs), that would check the site and report any issues with it. Here is one of the ways.
Installation on Fedora:
yum install linkchecker yum install python-tidy yum install python-cssutils
Example command line:
linkchecker -t20 --check-html --check-css http://mamchenkov.net
Obviously, check the manual of linkchecker for more options.