Comment by jazzypants

Qt widgets don't work together with GTK widgets, and nobody considers this a crisis. I'm pretty sure you can't use Unreal engine stuff in Unity. GUIs require a lot of stuff to compose together seamlessly, and it's hard to do that in a universal way.

HTMX achieves its composability by declining to have opinions about the hard parts. React's ecosystem exists because it abstracts client-side state synchronization, and that inherent complexity doesn't just disappear. When you still have to handle the impedance mismatch between "replace this HTML fragment" and "keep track of what the user is doing", you haven't escaped the complexity. You've just moved it to your server, and you've traded a proven, opinionated framework's solution for a bespoke one that you have to maintain yourself.

If anything, the DOM being a shared substrate means JS frameworks are closer to interoperable than native GUI toolkits ever were. At least you can mount a React component and a Vue component in the same document. They're incompatible with each other because they're each managing local state, event handling, and rendering in an integrated way. However, you can still communicate between them using DOM events. An HTMX date picker may compose better, but that's just because it punts the integration to you.