Variables

Less allows variables to be defined. Variables in Less are defined with an at sign (@). Variable assignment is done with a colon (:).

During translation, the values of the variables are inserted into the output CSS document.^[2]

@pale-green-color: #4D926F;

#header {
  color: @pale-green-color;
}
h2 {
  color: @pale-green-color;
}

The code above in Less would compile to the following CSS code.

#header {
  color: #4D926F;
}
h2 {
  color: #4D926F;
}

Mixins

Mixins allows embedding all the properties of a class into another class by including the class name as one of its property, thus behaving as a sort of constant or variable. They can also behave like functions, and take arguments. CSS does not support Mixins: Any repeated code must be repeated in each location. Mixins allows for more efficient and clean code repetitions, as well as easier alteration of code.^[2]

.rounded-corners (@radius: 5px 10px 8px 2px) {
  -webkit-border-radius: @radius;
  -moz-border-radius: @radius;
  border-radius: @radius;
}

#header {
  .rounded-corners;
}
#footer {
  .rounded-corners(10px 25px 35px 0px);
}

The above code in Less would compile to the following CSS code:

#header {
  -webkit-border-radius: 5px 10px 8px 2px;
  -moz-border-radius: 5px 10px 8px 2px;
  border-radius: 5px 10px 8px 2px;
}
#footer {
  -webkit-border-radius: 10px 25px 35px 0px;
  -moz-border-radius: 10px 25px 35px 0px;
  border-radius: 10px 25px 35px 0px;
}

Less has a special type of ruleset called parametric mixins which can be mixed in like classes, but accepts parameters.

#header {
  h1 {
    font-size: 26px;
    font-weight: bold;
  }
  p {
    font-size: 16px;
    a {
      text-decoration: none;
      color: red;
      &:hover {
        border-width: 1px;
        color: #fff;
      }
    }
  }
}

The above code in Less would compile to the following CSS code:

#header h1 {
  font-size: 26px;
  font-weight: bold;
}
#header p {
  font-size: 16px;
}
#header p a {
  text-decoration: none;
  color: red;
}
#header p a:hover {
  border-width: 1px;
  color: #fff;
}

Functions and operations

Less allows operations and functions. Operations allow addition, subtraction, division and multiplication of property values and colors, which can be used to create complex relationships between properties. Functions map one-to-one with JavaScript code, allowing manipulation of values.

@the-border: 1px;
@base-color: #111;
@red:        #842210;

#header {
  color: @base-color * 3;
  border-left: @the-border;
  border-right: @the-border * 3;
}
#footer {
  color: @base-color + #003300;
  border-color: desaturate(@red, 10%);
}

The above code in Less would compile to the following CSS code:

#header {
  color: #333;
  border-left: 1px;
  border-right: 3px;
}
#footer {
  color: #114411;
  border-color: #7d2717;
}

Sass

Both Sass and Less are CSS preprocessors, which allow writing clean CSS in a programming construct instead of static rules.^[5]

Less is inspired by Sass.^[6][3] Sass was designed to both simplify and extend CSS, so things like curly braces were removed from the syntax. Less was designed to be as close to CSS as possible, and as a result existing CSS can be used as valid Less code.^[7]

The newer versions of Sass also introduced a CSS-like syntax called SCSS (Sassy CSS).