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)

Isn't it because 'incorrect HTML' has been practically defined out of existence?

Opinion: the industry doesn't equate with reality. Pointy-haired MBAs with some PHP background would tell you otherwise, but there's a whole wide world out there.