Good read, thanks for that. I agree with the conclusion: there is no one-size-fits-all rule for interoperability.
The way I see it, it's ultimately about tradeoffs. I can only imagine what things would be like today if web browsers implemented a strict parsing of HTML and refused to render invalid pages. One possibility is hindered adoption of HTML by the masses. Another is that two vendors would disagree about the HTML spec and cause pages to be browser-specific. (Turns out this happened anyway :-))
