Doing HTML and JavaScript are commodities. Given the existence of standards, there's not a lot of room for innovative behavior. Any developments (faster, new HTML standards, etc) won't impact the market unless they're in one of the major browsers. Competition is good, but there's only room in the market for a handful of rendering engines. Maybe 3-5, tops.
Conversely, UI and human interaction with the browser is a place where there's room for an unbounded number of players. This is where the competition can actually be happening. And it's where I think the competition should be happening.
The idea of a browser as something that just renders HTML and executes JavaScript is kinda lame. I use Opera because I think it's also the browser's responsibility to include tab organization and search functionality and mouse gestures and flipping CSS in and out and synchronization. (And I like my mail integrated, but that's a personal thing.) And I want it to Just Work, instead of having to manage a half-dozen extensions.
I am guardedly optimistic about Vivaldi, because it's what I actually out of a browser. (Specifically, an updated version of Opera 12, which Opera 15+ is not.)