>I guess building an actual browser, from scratch
No one does this. For example all the major browsers reach out to an existing library to implement rendering for fonts. There is so mich complexity and already a solution to solve the problems and allow you to focus on something more important. There are benefits in standardizing on a single thing and having everyone working to improveia common base. Considering the actual rendering and functionality of the web is standardized the most exciting features kf a browser will be outside of the browser engine.