I'm using quickjs (rquickjs specifically) to bind GTK's C API to Javascript and adding a light DOM wrapper (get/setAttribute, add/removeEventListener, etc) that covers all the functionality Svelte needs. I've had to make a few modifications to the compiler (<10 lines total) to get binding and CSS working but so far it's been great.

Working in GTK with hot reload and instantaneous esbuild rebuilds is.... insane.