CSS Selectors

In CSS, selectors are patterns used to select DOM elements.

Here is an example of using selectors. In the following code, a and h1 are selectors:

a {
  color: black;
}

h1 {
  font-size 24px;
}

Cheat list of selectors

Selector Selects
head selects the element with the head tag
.red selects all elements with the ‘red’ class
#nav selects the elements with the ‘nav’ Id
div.row selects all elements with the div tag and the ‘row’ class
[aria-hidden="true"] selects all elements with the aria-hidden attribute with a value of “true”
* Wildcard selector. Selects all DOM elements. See bellow for using it with other selectors

We can combine selectors in interesting ways. Some examples:

Selectors Selects
li a DOM descendant combinator. All a tags that are a child of li tags
div.row * selects all elements that are descendant (or child) of the elements with div tag and ‘row’ class
li > a Difference combinator. Select direct descendants, instead of all descendants like the descendant selectors
li + a The adjacent combinator. It selects the element that is immediately preceded by the former element. In this case, only the first a after each li.
li, a Selects all a elements and all li elements.
li ~ a The sibling combinator. Selects a element following a li element.

Pseudo-selectors or pseudo structural classes are also useful for selecting structural elements from the DOM. Here are some of them:

Selectors Selects
:first-child Target the first element immediately inside (or child of) another element
:last-child Target the last element immediately inside (or child of) another element
:nth-child() Target the nth element immediately inside (or child of) another element. Admits integers, even, odd, or formulas
a:not(.name) Selects all a elements that are not of the .name class
::after Allows inserting content onto a page from CSS, instead of HTML. While the end result is not actually in the DOM, it appears on the page as if it is. This content loads after HTML elements.
::before Allows inserting content onto a page from CSS, instead of HTML. While the end result is not actually in the DOM, it appears on the page as if it is. This content loads before HTML elements.

We can use pseudo-classes to define a special state of an element of the DOM but don’t point to an element by themselves . Some examples:

Pseudo-class Selects
:hover selects an element that is being hovered by a mouse pointer
:focus selects an element receiving focus from the keyboard or programattially
:active selects an element being clicked by a mouse pointer
:link selects all links that have not been clicked yet
:visited selects a link that has already been clicked

Games

CSS Diner is a web game that teaches almost everything there is to know about combining selectors.

Additional reference

There are many more CSS selectors! Learn about them at CodeTuts, CSS-tricks.com, w3schools.com or at Mozilla Developer Network.