What does the ampersand (&) do in CSS pseudo-elements when combined with LESS?

Understanding the Ampersand (&) in CSS Pseudo-Elements

In CSS, pseudo-elements can be used to add styling to elements without modifying the content or structure of the HTML. When used in conjunction with pseudo-elements, the ampersand (&) character serves a specific purpose.

Consider the following CSS snippet, often seen in Twitter Bootstrap:

.clearfix {
  *zoom: 1;
  &:before,
  &:after {
    display: table;
    content: "";
  }
  &:after {
    clear: both;
  }
}

In this code, the ampersand (&) is used before the pseudo-elements :before and :after. This indicates that the styles applied to the pseudo-elements are only applicable when applied to the parent element with the class ".clearfix".

It's important to note that this syntax is not native to CSS. It originates from LESS, a CSS preprocessor. In LESS, the ampersand (&) allows for the nesting of selector modifiers. For instance:

.clearfix { 
  &:before {
    content: '';
  }
}

This code will compile to:

.clearfix:before {
  content: '';
}

The presence of the ampersand (&) ensures that the nested selector compiles to ".clearfix:before". Without it, the resulting code would be ".clearfix :before".

In summary, when used in conjunction with pseudo-elements in LESS, the ampersand (&) character allows for the nesting of selector modifiers, thus providing a convenient way to apply styles specifically to the descendants of a given element.

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