bind:

Svelte bind: is a two-way door. libwasm does not have a two-way door as a primitive. It has @prop (parent → DOM) and Slot (DOM → parent). bind:value={tone} is both: a host field tone, an @prop!"value" on the input named after that ident, a construct seed so the first paint matches the field, and an input / change Slot that writes the host field back. ComboForm.svelte is the proof for textarea and select. Number value is a double, not a string that you to!int in the handler — the extra parse would throw inside nothrow.

bind:this is a Handle. On an element it is filled in onMount, because construct has no handles yet. On a component it is the @child handle. bind:group on radios seeds checked from the host and writes back on change. bind:checked and bind:open are the same shape for checkbox and <details> / <dialog>. bind:innerHTML is @prop!"innerHTML". bind:files is a Handle. bind:indeterminate is ComboNest. bind:scrollY / scrollX on <svelte:window> call window().scrollY(). bind:paused, muted, and volume are media fields; volume is numeric.

Some DOM properties are getter-only. libwasm skips those assigns. dataset is a DOMStringMap. Layout metrics (clientWidth) and paused as a write are in that family. That is why {#each} index is int i and bind:clientWidth is a field, not @prop. Printing @prop!"dataset" looks closer to Svelte and does nothing at runtime.

bind:value={tone} and id={tone} cannot share a field. The attribute becomes id_. One write would otherwise clobber the other, and the input’s id would change when the value changed. construct seeds both from the host expression. The printed names are ugly and they are the reason the page still works.