WPF (even completely ignoring XAML), JavaFX both support declarative nesting with interesting databinding features to support it. They are based on retained scene graphs, however.
If you are interested in academics, I published such a system at ECOOP in 2006:
Declarative nesting of lifecycle-ful and stateful components without going full FRP.
I'm actually excited to learn about other frameworks that do this! Which do you have in mind?
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