I’ve used something similar quite a long time ago called Jade. Seems to be renamed to Pug. Here’s a nice writeup with some examples (not mine, found the webpage missing examples): https://www.sitepoint.com/jade-tutorial-for-beginners/
Also Pug is more of a templating engine, while this doesn't appear to have any interpolation / templating logic stuff.
The shorthand for classes/IDs seems like a good idea too, if there's a way to do it that improves readability (and seems less magical to the unfamiliar coder)
I'm a fan of higher expressiveness/minimalism, so perhaps I'm just biased in my preference, but I also appreciate how it removes the need to know/care about the inconsistent closing tag semantics of html.