Do's:
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Use your web stats to your advantage; If you're not making scalable tables, conform the site to your users' most popular resolution.

Use clear, simple fonts.

Don't be afraid to mix mediums. Cross pollenating you site with different web languages can often offer directions you didn't notice before.

Dont's
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Don't use Flash intro's without preloaders, skip links, or file sizes which will not load on a 56k connection within 2-3 seconds.

Don't embed tables in tables. This works on some browsers, others cough. Avoid as much as possible.

Never assume that everyone uses a current generation browser. One would be surprised how many people still browse on 4th and 5th generation browsers.