> What's your alternative that stay HTML spec compliant?

Again: you literally have a custom Javascript-like DSL in Datastar. Use that.

It's like the "it's just HTML" or "it's HTML-compliant" mantra somehow damages the brain, or something.

Edit: this custom JS-like DSL is so prominent and such a crucial part of Datastar, that it's referenced in the very first paragraph of reference: https://data-star.dev/reference

Oh, look, your HTML-spec-compliant thing in which it is apparently impossible to do anything outside data-* attributes somehow calls external functions, and updates signals, and reads signals, and does all sorts of things:

    <div data-signals-result>
        <input data-bind-foo 
           data-on-input="myfunction(el, $foo)"
           data-on-mycustomevent__window="$result = evt.detail.value"
        >
        <span data-text="$result"></span>
    </div>

But no! It's absolutely impossible to do something about data-on-click__window__debounce.500ms.leading