Kudos, Andrea, you really have some pretty cool projects running 👍

A little (slightly opinionated 😉) proposal (I do not know, whether this has already been discussed somewhere):

Now that uhml supports . and ? ...

<an-element an-attr="..." .aProp=${...} ?a-bool-attr=${...} onan-event=${...}/>

... wouldn't it make sense to go the extra mile and also add support for @... for events (similar to lit-html) as alternative to on...?
on... should of course continue to be allowed (I personally would declare on... deprecated and get rid of it in the major version after the next major version, but anyway).

<an-element an-attr="..." .aProp=${...} ?a-bool-attr=${...} @an-event=${...}/>

I guess that if the development of uhtml had been started in 2021 with all the current knowledge, there would not have been this ugly JSX-like on... convention (which makes sense in JSX but not in uhtml, or does it?)

Things would be at least a bit more compatible with lit-html .... and please compare the readabilty of

<sl-details summary="..." onsl-show=${...} onsl-hide=${...}>...</sl-details>

to

<sl-details summary="..." @sl-show=${...} @sl-hide=${...}>...</sl-details>