This is html building not js. It’s not any more verbose, in fact slightly less because no need for closing tags. Main difference is parens instead of angle brackets. Now you can use tools.
I do prefer lower case callables but that’s a minor nitpick, and “htpy” and other libs can do that.
Yes, it is HTML, and it loses all the benefits of HTML, by adding complexity of code and loss of structure. I'm uncomfortable with HTML and JS obfuscated by Python like that. I'm not using the word "verbose" as a character count comparison, but as an overall feel of weight when I see such code. It just takes me back to Java Swing, or ExtJS in the JavaScript world. It's not bad if it's the only way to describe an interface or layout. But there are better ways - HTML.
Please don't get my comments as criticism of the project itself, I think it's lovely and has a lot of merit. I've had to deal with the aftermath of these kind of things before, which makes me very aware of where it usually ends up at:
Devs in language X don't like Html/JavaScript/Y/Z, so they wrap it with language X until X is all there is. Then one day, the business realises they have a codebase nobody other than it's original creators can or want to deal with, and any change becomes a behemoth of a project. It always starts with the best of intentions.
The complexity and structure are near equivalent. S-expressions are an html precursor.
Python is one of the most popular languages and easy to read. People are more resilient than given credit.
I tend to agree with your closing comment about over abstraction in general, however you may have forgotten that a Jinja/html template is an abomination of conflicting concepts.
I’m more worried about the rest of this framework to be honest. :-D
I do prefer lower case callables but that’s a minor nitpick, and “htpy” and other libs can do that.