Even though your pages look good in your browser, that's no guarantee that they'll look or behave the same when somebody else views them. You can (and probably should) test them in several common browsers, but you might also consider running at least some sample pages through one of the available HTML validators.
CSS Validator from W3.org
HTML Validator from W3.org
CSE HTML Validator
Web Page Purifier
An amazing collection of validators at www.htmlhelp.org
Bobby assesses the accessibility of your pages and offers in-depth reports complete with suggestions for improvements. You can send Bobby a single page at a time over the web or download it to run on your own computer so you can test your entire site.
Apps that optimize HTML in one way or another:
HTML (Un)Compress by Jan Jacobs, available from various shareware archives
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