A good web standard can prevent most problems plagued by web sites significantly increase traffic, revenue and profit.
A good start for a Web standards check list written by Russ Weakley.
http://www.maxdesign.com.au/articles/checklist/
The Apple Human Interface Guidelines is a worth while reference as well.
http://developer.apple.com/library/mac/#documentation/UserExperience/Conceptual/AppleHIGuidelines/XHIGIntro/XHIGIntro.html#//apple_ref/doc/uid/20000957
The w3 validator is a good tool to quickly validate your html code on your web site to verify that the site is compliant as well.
W3 Markup Validation Service