Validate your pages to make sure that they are correct HTML. Some web authoring tool generate invalid HTML. Also check the Cascading Style Sheet markup, and verify that the links actually work:
Verify HTML code Verify CSS style markup Verify links with Link Valet
How to do Search Engine Optimization How To Use Google AdSense Within XML/XHTML
Measure the Google PageRank of your pages with some tool. This one seemed the most useful the last time I checked, but you might also try checkpagerank.net or mypagerank.net or webtoolhub.com
See how various browsers support or fail to support CSS, DOM, and other features on the QuirksMode.org compatability table.