Where so you start? If you have decided on using Python for the UI you have a handful of libraries each with their own documentation. To get started you need to choose one and then read the guides.
If you decide to use HTML you have a whole lot more resources. I'd argue you'll be able to make a frontend faster using this approach.
I understand the hesitation because of the complexity associated with JS. Maybe you're thinking about bundlers and minifiers when you think about writing a frontend with JS.
But you don't need that. You can create amazing user experiences with a plain HTML, CSS and a JS file.
1. It is inconsistent [1]
2. It does not have many features that programmers love and other languages have.
3. It has a community that holds different values from what other programmers value themselves.
If people want to continue development in JavaScript, they can. But since JavaScript has a monopoly on the (fairly large) 'frontend development for the web' market, there will always be attempts to find an alternative.
2. What are the missing JavaScript features that could be useful for web development?
3. I don't understand what you mean by this.
Finding alternatives are great. My point is that JavaScript/HTML/CSS are great and it's worth learning them if you want to work on frontends.
[1] https://stackoverflow.com/questions/66947264/why-do-python-s...
I disagree on the using something like this to start making a UI. When you know that your project requires a complex frontend, you're better of starting with HTML.