There’s nothing new about expressing XML in a significant-indentation format. Here’s Slim next to one from 2001.
html> head> title> My Home Page body> h1> Contact Details p> I can be contacted at a> href=mailto:email@example.com this address except when on vacation.
html head title My Home Page body h1 Contact Details p I can be contacted at a href="mailto:firstname.lastname@example.org" this address except when on vacation.
They’re quite similar. I find Slim a bit more comfortable in a ruby environment because it’s a bit more terse and uses two spaces like Ruby code does. It also is a templating language too. (In fact, I’m not sure there are converters going in both directions between XML/HTML and non-templated Slim. It could make a nice addition.)