It turns out it's really hard to get a UI Builder right with HTML/CSS. In particular, layout is tricky because DOM trees imply both bottom-up and top-down constraints. For example, text getting longer pushes down content beneath it, while at the same time, the text might be 50% of it's parent node's width. The first constraint is bottom-up: the text node influences it's parent and sibling DOM node's geometry, while the second constraint is top-down.

I'm working at Pagedraw (https://pagedraw.io/) which attempts to solve this problem. We make a UI builder for web. It pretty much works like all the UI builders of the past, but translates constraints into divs, flexbox, and other web standard html/css. We don't have the benefit other UI builders in the past have had of compiling into our own layout system, because we refuse to force our users to use a js-based layout library or take extra dependencies which could increase bundle size.

It's only available for React Web for now, but we look forward to supporting React Native, and maybe even Proton Native in the future!

Lastly to matchbok's point:

> NIBs in macOS/iOS are horrible for development and code review.

We hope that this is a problem with NIBs, and not with UI builders in general. We believe that it's because there are no NIB diff/merge tools, so we're building branch/diff/merge into Pagedraw as a first-class feature.