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Is Marko the ONLY framework with this feature? Surely there's some more.

I've been looking at this for months and AFAIK it's the only one that has it OOTB. If you find others, please let me know.

It can be implemented with React and others (I've done it with Svelte) but it's complicated. See this example on how to do it with Preact and Next:

Elder.js (Svelte framework) has partial hydration OOTB: https://github.com/Elderjs/elderjs