On the web side you need to synchronize the DOM, the visual state (which is mostly related to the DOM), the user's state, and the backend state using a protocol

On the web side you need to synchronize the DOM, the visual state (which is mostly related to the DOM), the user's state, and the backend state using a protocol that's stateless. Really, the browser side stuff is the real PITA. It's a hacked together piece of shit written by people who were too clever by half. And it's designed to be used/driven by less-than-skilled individuals. But in the end one big problem is there aren't a lot of ways to model a UI effectively. One given screen can have multiple states and behaviors that are state dependent, and the tools really aren't there. How do you specify "animation like x/y/z" on a screen? There are tools for designers, but they're don't really work when handing that off to development. What I've seen is people printing out prototype screens and annotating everything, but that's obviously janky.