nested css rules [duplicate]

That is not valid standard CSS, but it’s an example of nesting class declarations using Sass/SCSS or LESS and I believe other CSS extensions out there, which expand it to something like this (which is valid CSS), before serving it to the browser to use:

div.a {
  color: red;
}

div.a div.b {
  /* Inherits color from div.a */
  font-weight: bold;
}