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A descendant selector in CSS is any selector with white space between two selectors without a combinator. Here’s some examples:

ul li {  }
header h2  {  }
footer a  {  }
.module div {  }
#info-toggle span  {  }
div dl dt a  {  }

Take ul li { } for example. It means “any list item that is a descendant of an unordered list.”

Descendant means anywhere nested within it in the DOM tree. Could be a direct child, could be five levels deep, it is still a descendant. This is different than a child combinator(>) which requires the element to be the next nested level down.

To illustrate, div span { } will match:

  <span>I will match</span>
       <span>I will match too</span>

You probably shouldn’t worry about it very much, but the decedent selector is pretty “expensive” – meaning hard/slow for rendering engines to figure out and do stuff with. MDN:

The descendant selector is the most expensive selector in CSS. It is dreadfully expensive—especially if the selector is in the Tag or Universal Category.

But only in comparison to other selectors. It’s still blazingly fast and you’ll probably never notice it unless you go crazy.

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