When I started development, I didn't know that DOM controls could overlap, let alone lay over a WebGL canvas :) I had zero HTML (and general web development) knowledge at the time. Since I have some experience doing this kind of stuff, instead of diving into the scary DOM details, I took the path of least resistance for me (begun implementing a GUI on a nice, clean drawing surface).

Joking aside, it didn't start as a fully-fledged UI. Some scrolling text strings to show a message log first, the ability to move them out of the way later, then implementing a proper window container for them, then introducing the concept of a 'widget', then getting really excited and thinking about a layout system to be able to present multiple widgets per window... that's the way it usually goes :) The GUI turned out a lot more demanding than the terrain engine itself, both in development time as well as rendering load.

In the end, it's all about learning!