[kbutler]

Be strict in what you produce and liberal in what you accept.

Strictness works well when you have a small number of active producers and consistent, constantly renewed content.

The web is the opposite - a huge number of producers and widely varying content, some of which is 1-2 decades old and will never be updated.

You can design and build a great, strict HTML parser - it just won't work well for a significant portion of the www.

And if you make one that is popular enough, you can change the direction of evolution of the web! (See mobile safari and flash)