Sure, lots of UI designers do. But it isn't a process that scales. If you're trying to define design requirements for a product that's very large or distributed (i.e. Netflix), manually drawing pictures of a UI is simply too slow.

Some designers have taken to writing CSS, and some even prefer it as their main design tool. I think that's awesome, but CSS exposes lots of implementation details that are only relevant to the web. I'm trying to create a declarative syntax that feels as easy as CSS, but exposes lots of functionality that would normally require javascript. And it will be abstracted from the platform, so this would serve not just web designers but also iOS/Android/TV/etc.