This is an apples-oranges comparison, unless things have changed drastically since the last time I worked on L4 (about 10 years ago). L4 is very secure and easy to reason about. But that's because it doesn't really do anything. It makes a lot of sense as a platform to build a general purpose OS on, and as a bottom layer for what would otherwise be a unikernel. But you'd run a browser on top of something that itself runs on seL4, not on seL4 itself.