Comment by potamic

Check out the list of all CSS specifications [1], and then open any one of them and see how lengthy and elaborate each is. Then do the same for each version of the spec published over the last thirty years. Before you can start, you must read and understand all of this at a great level of depth. Still, specifications never tell the complete story. You must be aware of all the nuances that are implied by each requirement in the spec and know how to handle the zillion corner cases that will crop up inevitably.

And this is just one part. Not even considering the fully sandboxed, mini operating system for running webapps.

[1] https://www.w3.org/Style/CSS/specs.en.html