Reactive HTML but it’s almost entirely JavaScript, or am I misunderstanding something?
It's JavaScript for the interactive/reactive parts, but the notebook is implemented in a self-contained HTML file: it handles everything from document structure to editing and display. It's also not dependant on 3rd party software (par browser), and is as durable, portable and easy-to-use as any HTML is.
It imports the ObservableHQ runtime in one of the first code blocks.