Parsing text can be hardly parallelized. Overall it will increase the price of your phone so it can show that webpage fast enough, which will be a cost of both CPU and battery. I have a shitty bi core smartphone, and it really doesn't like webpages. When you look at the source of a page, there is no justification for all of this, unless you like complexity.
I did not want to talk about HTML on smartphones, since it already seems to be non existent since apps replaced HTML, but do you really think we should have apps instead of webpages? Apps are less open. I really think HTML parsing is power intensive, and that you could actually increase autonomy and speed on HTML, instead of just having bigger batteries. Seems to be the same argument than fuel efficiency regulations.
Maybe I say all this because I like minimalism, but at the end of the day, most core formats are designed with acceptable performance in mind. In my opinion, HTML doesn't have acceptable performance.