I love the ability of less.js to make nested rules. For example
.A{ .B{ width:50px; } }
which results in
.
It's as simple as this:
.A { > .B { width: 50px; } }
Another related question: Immediate Child selector in LESS
Some documentation: http://lesscss.org/features/#features-overview-feature-nested-rules (doesn't actually include relevant example)