The HTML 4.01 standard (not always easy reading, but definitive on what is legal and illegal HTML).
Tutorials and Style Recommendations for writing Web pages
WebMonkey, a widely-used source of information for Web developers, including handy HTML references
ICRA, the Internet Content Rating Association, is an international, independent organization that provides a rating system for Web pages. The author of the page fills out a form rating the page for nudity, sexual activity, language, violence, etc. and parents can set the Web browsers on their home computers to allow or deny access to sites based on the sites' ratings and the parents' standards and preferences.
NetParents was founded to give parents easy access to information and tools for controlling their children's access to the Net. It does not advocate any particular standards, nor any particular filtering software, but only the general notion that the power to limit content belongs with parents, enforced at the receiving end, rather than with government, enforced at the publishing end.
<noframes>tag to tell lynx users how to get at the content. And so on.