The most important factor is to make it clean code, validated by W3C. Websites with clean code display correctly in all the major browsers like Internet Explorer, Firefox, Safari, Opera and beyond...
Search engines want to give users a good experience to keep their loyalty so when it finds two similar sites, the one with the clean code wins.
A full explanation on how to clean up for code and have SEO HTML can be found on the following three pages:
I'm Tim Hill, a Search Engine Optimisation and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.