Top factors for creating SEO-friendly HTML to improve search engine placement (according to this article):

1. Keyword Use in Title Tag.
2. Keyword Use in Body Text.
3. Relationship of Body Text Content to Keywords (Topic Analysis)
4. Keyword Use in H1 Tag
5. Keyword Use in Domain Name
6. Keyword Use in Page URL
7. Keyword Use in H2, H3, H4, H5, H6 Tags
8. Keyword Use in ALT Attributes and Image Titles
9. Keyword Use in Bold/Strong Tags (and Em)
10. Keyword Use in Meta Description Tag

Some other ideas for improving SEO performance on your sites:

1. Choose a good domain name, ideally with keywords embedded in it. Own it for years, too, so that you do not appear to be fly-by-night.
2. Have a sitemap for Google, especially if your pages are dynamically generated.
3. Move your navigation code downward. Pretend that a search engine spider “loses interest” if your page is too long. If all of your navigation and header code is at the top of your HTML file, the spider may lose interest before it gets to the actual content. Using CSS positioning, put your navigation code below the main content in the BODY section.
4. Keep your site dynamic. Keep adding content, and updating old pages on your site.
5. Check your site for errors, and fix them. Broken links may creep in, other issues. Google likes error-free sites – it shows you care. :)
6. Make sure the major search engines know about your site. Submit it to Google, Yahoo, MSN, and DMOZ.
7. Get well-ranked pages to link to your site.