I think this is a terrible idea for some reasons.

1. If you're building a web page and considering animations/class toggles there's a very high chance you have at least a little knowledge of JavaScript (or jQuery, probably) – implementing these examples in JS is trivial.
2. Adding another abstraction layer using another library that compiles to JavaScript just for some class toggles? Again, there's high chance your codebase already has JavaScript code for any other purpose (forms, etc). That would mean scatter the codebase without any reason.
3. Using <code> tags is the worst part. You're abusing HTML elements and that's a bit too hacky to be the foundation of your library.

Just remember this and you don't need it:

$('.element').on('click',function(){ $('.otherElement').toggleClass('hide'); });

why the hell is it <code> instead of <script>?

Shorthand.js is something similar (but uses <script> instead of <code>, amongst other benefits).

Agree with the reservations discussed here, but it’s nevertheless a novel idea and would be handy for prototyping, if nothing else.

Now that's cool. I can see myself using it instead of jQuery for interactions.

<a onclick="document.getElementById('toggle').className = 'visible';">

I’m using this instead of jQuery. It’s simpler. The only downside is you have to use an #ID.

[moved into the reply to the comment by Ivan Prignano]

The idea is cool, but I can see some problems using this in production environments.

I think the uilang.com site is really beautiful but this is kinda pointless - it's basically just as easy to do all this stuff with jQuery

I get why he did it after reading the medium post and find the idea kind of interesting. I guess I would still feel uncomfortable teaching someone DOM manipulation with a different syntax that might ultimately confuse them.

I worked for a short time as a teacher's assistant at a private college last year, the foundations of Javascript were taught as vanilla JS before introducing any libraries or frameworks.

I guess I kind of think this is the right approach to teaching design and development but I might be wrong.

I think it’s curious that the examples show HTML, CSS, as I thought they would be quite tricky to pick up too, with their precise syntax and required boilerplate.

I reckon this is pretty cool after seeing the thoughts behind it at https://medium.com/@bdc/the-educational-side-of-uilang-92d39da94c13

Remember, not everything has to be a final, that’s it, I have so-ouullved the prob-lemmmmm! kind of solution, so I’ve got to admire that this was remembered as a particular hard hurdle, using JavaScript, and this is one way way it could made easier. Like the way the creator says that if this can be used as training wheels to get someone moving that can be then knocked off after a few weeks, that’s a pretty worthwhile idea!

I think if you consider it to be throwaway code, it could be a very valuable prototyping tool.

There's nothing wrong with being wasteful in the design process. If you're worry about making it production ready, chances are, you'll be more precious about your work and be afraid to scrap it.

I appreciate the sentiment behind this but I guess I just think that HTML/CSS/JS/jQuery is easy enough for a designer to learn...

I believe for me it's cool.