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.
In principle those external dependencies could be rolled together into one document and produce something truly freestanding. Once one decides to inline everything, opportunities arise to produce something simpler and more coherent.