Not that 80
In his post Parroting Pareto, Jeremy Keith says that HTML5 needs to cover cases that “fall far outside the 80%-90% curve”, in particular accessibility. “By their very nature, accessibility concerns are not going to affect the majority of users. That doesn’t mean they can be dismissed.”
My understanding of applying the 80/20 rule to the design of HTML5 is that the “80” isn’t about 80% of users. It is about (proverbial) 80% of authoring cases. That is, it doesn’t make sense to support (for accessibility or otherwise) things that people would only publish very rarely if engineering support for the rarity would complicate the implementation of the language significantly.
See Hixie’s email to the HTML WG on the topic.