less.js/test/css/extend-nest.css

.sidebar,
.sidebar2,
.type1 .sidebar3 {
  width: 300px;
  background: red;
}
.sidebar .box,
.sidebar2 .box,
.type1 .sidebar3 .box {
  background: #FFF;
  border: 1px solid #000;
  margin: 10px 0;
}
.sidebar2 {
  background: blue;
}
.type1 .sidebar3 {
  background: green;
}