Looks like it's done using standards-based web components[0]. The page says these components don't require any existing JavaScript framework; because web component support is built-in to the browser.
Nice to see devs picking up web components.
[0]: https://developer.mozilla.org/en-US/docs/Web/API/Web_compone...
I remember toying with Polymer circa 2014, for some reason the word "transclusion" jumps into my mind, I remember being excited about it at the time. I barely remember what it means today though.
Polymer still haunts me to this day. It never made sense. It was literally designed to be deprecated. It's a big nasty polyfill for web components and it had/has a huge perf overhead. Not to mention it's ergonomics are just bad.