Nesting CSS classes

Not possible with vanilla CSS. However you can use something like:

• Sass

Sass makes CSS fun again. Sass is an extension of CSS3, adding nested rules, variables, mixins, selector inheritance, and more. It’s translated to well-formatted, standard CSS using the command line tool or a web-framework plugin.

Or

• Less

Rather than constructing long selector names to specify inheritance, in Less you can simply nest selectors inside other selectors. This makes inheritance clear and style sheets shorter.

Example:

#header {
  color: red;
  a {
    font-weight: bold;
    text-decoration: none;
  }
}

Update 1: There is a CSS3 spec for CSS level 3 nesting. It's currently a draft. https://tabatkins.github.io/specs/css-nesting/

Update 2 (2019): We now have a CSSWG editors draft https://drafts.csswg.org/css-nesting-1/

Update 3 (2022): We now have a W3C First Public Working Draft https://www.w3.org/TR/css-nesting-1/

If approved, the syntax would look like this:

table.colortable {
  & td {
    text-align:center;
    &.c { text-transform:uppercase }
    &:first-child, &:first-child + td { border:1px solid black }
  }
  & th {
    text-align:center;
    background:black;
    color:white;
  }
}

.foo {
  color: red;
  @nest & > .bar {
    color: blue;
  }
}

.foo {
  color: red;
  @nest .parent & {
    color: blue;
  }
}