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If you've never validated before, be prepared for the possiblity of tons of errors. On the plus side, most will turn out to be the same error, repeated many times. Also, most will be easily repaired.
Be sure that you declare a DTD, and specify character encoding.
The great thing about the HomeSite validators is you can set the level of compatibility. For example, if you're just updating an old site and the customer doesn't want to pay to bring it up to at least 4.01 transitional (they seldom do, if they can't **see** the difference, in their minds there **is** no difference) you can set the validator level to html 3.2.
Pre-validating is a good step, one that saves you time and saves w3c bandwidth. A time may come when that validator will no longer be "free."