Mastering CSS Selectors

Why Do We Need to Learn About Selectors?

Completing a stylesheet is like having a little war between selectors, you are constantly overriding previously defined styles with new ones:

1/* You first might want all the H2 tags to be font-size: 14px; and color: blue; */
2h2 {
3    font-size: 14px;
4    color: blue;
5}
6
7/* But then you have a very particular page where the background is also blue, so you need your headings to be white */
8
9h2 {
10    color: white;
11}

This happens all the time and, at some point, it can be challenging to override previous styles. You must organize your styles properly and start from the least specific to the most specific.

These "very specific" selectors will help you a lot. They will be your best weapon when fighting your styles war!

📺  Here is a super cool video (3:40 min) explaining specificity.

The Child Selector

1#someDiv > p {
2    color: blue;
3}

This statement takes the paragraph tags that are children of the div and turns them blue. Note that it only works for the children of that div, not necessarily for all of the descendants. Let’s explore this further with the following example.

The Adjacent Sibling Selector

1p + p {
2    color: red;
3}

We used the adjacent sibling selector to change the second and third paragraphs to red. This seems very tricky, doesn’t it? Instinctively, we would expect the first paragraph to be red as well. After all, the first paragraph is on the same level as the next two, and, has siblings.

However, this selector only applies to elements that are preceded by something else. In this instance, only the paragraphs preceded directly by a sibling paragraph will be targeted.

The first paragraph in the list is preceded by the div, so it remains unchanged.

The Almighty Asterisk

1#someDiv * {
2    color: red;
3}

The previous CSS code turns red every element inside a specific div, this includes items such as links, that have a default color set to something else and wouldn’t be affected by simply targeting the div.

1div * p {
2    color: red;
3}

You can take this as far as you want; the following targets the "grandchildren" of the div. You will find this chaining method used frequently in CSS debugging tricks.

Attribute Value Selector

1a[href='http://4geeksacademy.com/'] {color: blue;}

If we want to change the font color of the "4Geeks" link, we could use pseudo selectors. However, doing so would assume that the list stays in that order, and, browser support isn’t the best. Instead, what we can do is use an attribute selector to target the specific href that we’re interested in.

Arbitrary Substring Attribute Value Selector

1div[id*='section'] {color: red;}

The following code targets any div with the word "section" in the title. It can be "section3" or "section-Four", it doesn’t matter. As long as it contains the indicated string, the subsequent styles will apply.

Pseudo Class Selectors

Link-related Pseudo Selectors

1a:link{color: green;}
2a:visited{color: yellow;}
3a:hover{color: blue;}
4a:active{color: red;}

You can assign a different color to any link on the website, depending on its status:

• :link default color, before clicking on it.

• :visited after clicking the link.

• :hover when the cursor is over the link.

• :active when the cursor is clicking on the link.

Input-related Pseudo Selectors

1input {padding: 5px;}
2input:disabled {
3    background: #ddd;
4    color: #949494;
5    border: 2px solid black;
6}
7input:focus {font-size: 2em;}
8input:enabled {border: 2px solid black;}

It is very important to take enough time to style our forms. Styling is the best way to tell the user that one input is either disabled, checked, or that the cursor is over a particular input.

Position-based Pseudo Selectors

1#myUL li:first-child {background: blue;}
2#myUL li:nth-child(3) {background: orange;}
3#myUL li a:first-of-type {background: green;}

You can apply styles to elements based on their position.

Here is a List of the Most Used Pseudo-Classes:

Selector	Example	Example description
:active	a:active	Selects the active link
:checked	input:checked	Selects every checked <input> element
:disabled	input:disabled	Selects every disabled <input> element
:empty	p:empty	Selects every <p> element that has no children
:enabled	input:enabled	Selects every enabled <input> element
:first-child	p:first-child	Selects every <p>element that is the first child of its parent
:first-of-type	p:first-of-type	Selects every <p> element that is the first <p> element of its parent
:focus	input:focus	Selects the <input> element that has focus
:hover	a:hover	Selects links on mouse over
:in-range	input:in-range	Selects <input> elements with a value within a specified range
:invalid	input:invalid	Selects all <input> elements with an invalid value
:lang(language)	p:lang(it)	Selects every <p> element with a lang attribute value starting with "it"
:last-child	p:last-child	Selects every <p> element that is the last child of its parent
:last-of-type	p:last-of-type	Selects every <p> element that is the last <p> element of its parent
:link	a:link	Selects all unvisited links
:not(selector)	:not(p)	Selects every element that is not a <p> element
:nth-child(n)	p:nth-child(2)	Selects every <p> element that is the second child of its parent
:nth-last-child(n)	p:nth-last-child(2)	Selects every <p> element that is the second child of its parent, counting from the last child
:nth-last-of-type(n)	p:nth-last-of-type(2)	Selects every <p> element that is the second <p> element of its parent, counting from the last child
:nth-of-type(n)	p:nth-of-type(2)	Selects every <p> element that is the second <p> element of its parent
:only-of-type	p:only-of-type	Selects every <p> element that is the only <p> element of its parent
:only-child	p:only-child	Selects every <p> element that is the only child of its parent
:optional	input:optional	Selects <input> elements with no "required" attribute
:out-of-range	input:out-of-range	Selects <input> elements with a value outside a specified range
:read-write	input:read-write	Selects <input> elements with no "readonly" attribute
:required	input:required	Selects <input> elements with a "required" attribute specified
:root	root	Selects the document’s root element
:target	#news:target	Selects the current active #news element (clicked on a URL containing that anchor name)
:valid	input:valid	Selects all <input> elements with a valid value
:visited	a:visited	Selects all visited links

🔗 Great reading about CSS Selectors: The 30 CSS selectors you must memorize