For a long time, Bootstrap was the flagship Less project, but they've recently switched to Sass. Some of the increasing momentum in Sass is probably due to libsass, a C-based Sass compiler that can be used without depending on Ruby. (And is also much faster.)

Just FYI: Neither Sass nor Less should be written in all-caps. See the respective websites.

It's worth noting that libsass's source map support is still ironing out bugs (i.e. https://github.com/sass/libsass/pull/1463), though it seems that the compilation itself is relatively solid. My team finds that the time saved in compilation for a Flask app is worth source maps sometimes not working. Your mileage may vary, though.

libsass is - understandably - a few versions behind the Ruby implementation. Most of the project teams I've been on working with Sass have opted for the Ruby implementation as a consequence.

The ruby version is on feature freeze until libsass catches up.

citation now I'm on desktop:

> In fact, Ruby Sass will not release any new features until LibSass catches up. Once it does, there will be feature parity between the two moving forward. Soon, we’ll have the blazing speed of LibSass with all the features of Ruby Sass!

http://sassbreak.com/ruby-sass-libsass-differences/

Looks like there's a more discussion of Sass on Stackoverflow than Less - http://www.arepeopletalkingaboutit.com/tags/less,sass

For what it's worth, Bootstrap 4 is switching from Less to Sass - http://blog.getbootstrap.com/2015/08/19/bootstrap-4-alpha/

Usually it doesn't matter what it's written in, because unless a company wants their site to look like a template, everything will be overwritten. And the value of the base idioms will still be retained for the dev team.

Since this looks like it's to help create a consistent look & feel for Office UI, I'm not sure why they chose LESS. :) Most likely that's what they were familiar with, and maybe they wanted to keep dependencies low. But there's always Stylus [1], and it's at least as powerful as SASS. Plus it has better macro integration, which is very convenient when you forget to call @mixin.

    Sytlus "border-radius: 5px;" vs SASS "@include border-radius(5px);"

The main reason LESS isn't desirable is it's very restricting & has no lexical scoping, which makes writing plugins such as spriting very awkward to write.

[1] https://learnboost.github.io/stylus/

I'm guessing that the whole point of this framework is that you can build internally-used webapps that feel like Office. So in that case, you would absolutely want your code to 'look like a template'.

Neither, the cool kids are using Radium https://github.com/FormidableLabs/radium :-J