Thanks, are they able to render custom elements that have shadow DOM, including the styles?

no, it's a way to share the same templating syntax on the client and server but isn't a full server side rendering solution. It's pretty fast though, just about the fastest templating engine out there last time I checked.

Using the CLI, can templates import other html bits (classic header/content/footer for example) ?

yes, you can do something like `HTMLing.registerElement('foo-bar', {})` and then when you use `<foo-bar />` in another template it will render it. I should note that I'm not maintaining this library very well because I've mostly switched to React, but it does work.