It's a *mammoth* task to write one from scratch. By *from scratch* I mean writing the HTML / JS rendering engine, UI, interaction, video handling etc. all from scratch. Really, it's huge. There's a reason why those browsers have had many hundreds of people working on them for many years...
It's comparatively easy to take a HTML rendering engine and stick some sort of UI on it, but then you have to ask what you really gain from that?