As a non-programmer (UX), I used to watch Polymer with so much interest, since it had HTML tag-like syntax. I copy paste component codes & construct an UI. There was simplicity in <googlemaps lat="89.2" long="123.2" draggable="false"></googlemaps>
Now with lit-html, it went from unique HTML/WebComponent for non-coders, to another one of the JS frameworks out there.
If <google-maps> was rewritten in LitElement, you'd still be able to write:
<google-maps lat="89.2" long="123.2" draggable="false"></google-maps>Unfortunately this breaks down as soon as you start to break up the page into sections (you end up with a component that needs a "layout" to provide surrounding <html><body>(...) etc).
Ironically, now with html5 being rather loose and fast, you might actually get a decent preview of such a widget again... But it's still not right for "web applications" as opposed to "hypertext applications".
Full applications need full gui widgets - and html isn't a good fit.
I think one difference between lit-html and React/other JS frameworks is that lit-html knows which portions of its template are static, and can avoid reconciling static portions. This is the same optimization as what Svelte can do, but without the need to change your toolchain.
Disclaimer: I work at Google, but on a completely unrelated team.