To be honest, most webmasters don't worry about validating
their code as often as they should. Running validation through
a service like: http://validator.w3.org/ is a great way to
make sure that your website is readable by the search engines.
Validated code means that all the stuff that goes on behind
the scenes is laid out the way it should be. If it's all good,
the spiders don't miss things.
