There’s nothing new about expressing XML in a significant-indentation format. Here’s Slim next to one from 2001.
Simple Outline XML (2001)
html>
head>
title> My Home Page
body>
h1> Contact Details
p> I can be contacted at
a> href=mailto:me@myplace.net
this address
except when on vacation.
Slim (2010)
html
head
title My Home Page
body
h1 Contact Details
p I can be contacted at
a href="mailto:me@myplace.net" 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.)