Maintainer of lit-html here, which uses tagged template literals in JavaScript extensively.

This looks really great! It's almost exactly like JavaScript tagged template literals, just with a fixed tag function of:

    (strings, ...values) => {strings, values};

It's pretty interesting how what would be the tag function in JavaScript, and the arguments to it, are separated by the Template class. At first it seems like this will add noise since it takes more characters to write, but it can make nested templates more compact.

Take this type of nested template structure in JS:

    html`<ul>${items.map((i) => html`<li>${i}</li>`}</ul>`

With PEP 750, I suppose this would be:

    html(t"<ul>{map(lambda i: t"<li>{i}</li>", items)}</ul>")

Python's unfortunate lambda syntax aside, not needing html() around nested template could be nice (assuming an html() function would interpret plain Templates as HTML).

In JavaScript reliable syntax highlighting and type-checking are keyed off the fact that a template can only ever have a single tag, so a static analyzer can know what the nested language is. In Python you could separate the template creation from the processing possibly introduce some ambiguities, but hopefully that's rare in practice.

I'm personally would be interested to see if a special html() processing instruction could both emit server-rendered HTML and say, lit-html JavaScript templates that could be used to update the DOM client-side with new data. That could lead to some very transparent fine-grained single page updates, from what looks like traditional server-only code.