"Be liberal in what you accept, and conservative in what you send." - http://en.wikipedia.org/wiki/Robustness_principle
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 :-))
Is the HTML5 spec better in terms of interop and compatibility than the previous ones? http://www.tbray.org/ongoing/When/201x/2010/02/15/HTML5