I agree with Tim Bray that there are indeed exceptions to Postel's Law.
As to Mark's post on this issue and others, I want to know what he is smoking. Right after dismissing a suggestion because not everyone can set HTTP headers, he claims all HTTP headers can be replicated at document level using HTML meta tag because the HTTP spec said so. Even if it did (it doesn't), anyone with at least one foot on the ground should know better than to expect the reality to support that sort of broad requirement. Yeah, I want some of what he is smoking.
An out-of-the-blue comments about intelligence:
Being highly intelligent just means there is a powerful engine under the hood. What one does with the car is an entirely different matter. And, of course, you need gas to power it. What powers your engine?