Constantly moving targets don’t help. CSS, HTML, and JavaScript add new features way too quickly. Between supporting new shiny stuff and crusty old stuff there’s so much bloat to keep up with. It’s taking huge efforts to design and tune these rendering engines, so there are only a handful of efforts now backed by major players (Google, Apple, Microsoft, and Mozilla). Even Microsoft threw in the towel on their own engine and started using Google’s Chrome because it was cheaper and easier.