What Does the Double Colon (::) Mean in CSS

What does the double colon (::) mean in CSS?

It means pseudo element selector. It means the element to the right doesn't exist in the normal DOM, but can be selected.

A pseudo-element is made of two colons (::) followed by the name of the pseudo-element.

Source

It was originally only a single colon, but was changed to differentiate it from pseudo classes (like :hover, :first-child, :not etc). It's best to use : for before and after pseudo elements since the single colon has better browser support, namely in earlier IE versions.

What is the difference between pseudo-classes and pseudo-elements?

From https://developer.mozilla.org/en-US/docs/Learn/CSS/Introduction_to_CSS/Pseudo-classes_and_pseudo-elements

Pseudo-class :

A CSS pseudo-class is a keyword, preceded by a colon (:), added to the end of selectors to specify you want to style the selected elements, and only when they are in certain state. For example, you might want to style an element only when it is being hovered over by the mouse pointer, or a checkbox when it is disabled or checked, or an element that is the first child of its parent in the DOM tree.

Examples:

• :active
• :checked
• :nth-child()
• :first
• :hover

Pseudo-elements ::

Pseudo-elements are very much like pseudo-classes, but they have differences. They are keywords, this time preceded by two colons (::), that can be added to the end of selectors to select a certain part of an element.

Examples:

• ::after
• ::before
• ::first-letter
• ::first-line
• ::selection
• ::backdrop

As stated by @stephanmg:

In practice ::before is used as :before and ::after is used as :after
because of browser compatibility. Both are pseudo-elements, but may
look like pseudo classes. This might be confusing if you read CSS
code.

Should I use single colons (:) or double colons (::) for before, after, first-letter and first-line pseudo-elements?

For what it's worth, Selectors 4 now explicitly instructs¹ authors to use double colons for all pseudo-elements, including CSS1 and CSS2 pseudo-elements, going forward (emphasis mine):

Because CSS Level 1 and CSS Level 2 conflated pseudo-elements and pseudo-classes by sharing a single-colon syntax for both, user agents must also accept the previous one-colon notation for the Level 1 & 2 pseudo-elements (::before, ::after, ::first-line, and ::first-letter). This compatibility notation is not allowed any other pseudo-elements. However, as this syntax is deprecated, authors should use the Level 3+ double-colon syntax for these pseudo-elements.

This means that the only appropriate use of the single-colon syntax today is if you absolutely require legacy browser support — the only browser that matters here is IE8 and older. If you don't, you should use the double-colon syntax for the sake of consistency with newer pseudo-elements which will only accept double colons. Besides, it's quite pointless to use the single-colon syntax if, for instance, you're going to apply properties that aren't supported by IE8 anyway, such as border-radius or box-shadow, to your ::before and ::after pseudo-elements.

I'd like to believe that Selectors 3 at the very least implied this in its statement that the single-colon syntax does not apply to any newer pseudo-elements, but having this sort of thing stated in black and white never hurt anybody and it's good to know that the upcoming standard does just that.

Also, there is absolutely no reason to write duplicate rules with both notations in the same stylesheet (e.g. :before, :after { ... } ::before, ::after { ... }), as no browser in existence supports the new syntax without supporting the older one.

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¹ _{I say this fully aware that it probably didn't state this yet at the time this question was asked — the May 2013 WD certainly did not.}

What does this operator (::) in CSS mean?

It indicates that what follows is a "pseudo-element". From the CSS Selectors level 3 spec:

A pseudo-element is made of two colons (::) followed by the name of
the pseudo-element.

And a pseudo-element creates an "abstraction about the document tree":

Pseudo-elements create abstractions about the document tree beyond
those specified by the document language. For instance, document
languages do not offer mechanisms to access the first letter or first
line of an element's content. Pseudo-elements allow authors to refer
to this otherwise inaccessible information. Pseudo-elements may also
provide authors a way to refer to content that does not exist in the
source document (e.g., the ::before and ::after pseudo-elements give
access to generated content).

For example, the ::webkit-scrollbar pseudo-element provides a mechanism to refer to the webkit scrollbar, which would be otherwise inaccessible. Another example: the ::first-letter pseudo-element provides a way to refer to the first letter of an element (if it is not preceded by any other content).

Should I use single or double colon notation for pseudo-elements?

Do not use both combined with a comma. A CSS 2.1 compliant (not CSS3 capable) user agent will ignore the whole rule:

When a user agent cannot parse the selector (i.e., it is not valid CSS 2.1), it must ignore the selector and the following declaration block (if any) as well.

CSS 2.1 gives a special meaning to the comma (,) in selectors. However, since it is not known if the comma may acquire other meanings in future updates of CSS, the whole statement should be ignored if there is an error anywhere in the selector, even though the rest of the selector may look reasonable in CSS 2.1.

http://www.w3.org/TR/CSS2/syndata.html#rule-sets

You could however use

.foo:after { /*styles*/ }
.foo::after { /*styles*/ }

On the other hand this is more verbose than necessary; for now, you can stick with the one-colon notation.

Why are there two colons here? span::before

It's a pseudo-element, as defined by the CSS Selectors Level 3 spec:

The ::before and ::after pseudo-elements can be used to describe generated content before or after an element's content.

It is effectively the same as the single-colon syntax defined by the level 2 spec. The level 3 spec introduces an extra colon to differentiate between pseudo-elements and pseudo-classes (which use a single colon).

Both syntaxes will work in newer browsers, but older browsers will not recognise the newer :: style.

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For even more detail, you can look at the grammar from the level 3 spec, which states:

'::' starts a pseudo-element, ':' a pseudo-class

CSS :: meaning

From the MDN:

Sometimes you will see double colons (::) instead of just one (:).
This is part of CSS3 and an attempt to distinguish between
pseudo-classes and pseudo-elements. Most browsers support both values.

So, when you want to use pseudo-classes like :hover, :first-child etc, use a single colon. If you want to use pseudo-elements, like ::before, ::after, ::first-letter and so on, use double colons.

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One more note: the W3C states that browsers should only accept the :: notation for pseudo-elements introduced in CSS 3, so you should follow the recommendations above :)

What does :: and ~ do in css?

The tilde character (~) is the siblings selector

h2 ~ p { color:red; }

for example would make the paragraphs red in the below code

<h2>Heading</h2>
<p>The selector above matches this paragraph.</p>
<p>The selector above matches this paragraph.</p>

the :: is used for ::before and ::after pseudo-elements which together with the content: allow you to put, for example, an icon before every link

a::before { content:url(link.png); }

CSS :: vs : -- pseudo-element vs pseudo-selector?

Pseudo-classes (:) allow you to style the different states of an element e.g. when you hover over it, when it is disabled, when it is the first child of its parent, etc.

Pseudo-elements (::) allow you to style different parts of an element e.g. the first line, the first letter, inserting content before or after, etc.

Originally they all used a single colon, but CSS3 introduced the double colon to separate the two.

when to use pseudo-classes and when to use pseudo-elements selectors in CSS

The single colon syntax is an older implementation. In general, there are pseudo-elements :: & pseudo-classes :, and they are not identical. In this case though, browsers still support the outdated single-colon syntax.

This means that in your example with :before/::before, it will not make a difference to the outcome, but in general you should use the double colon syntax, because before & after are pseudo-elements, not pseudo-classes.

Read more on MDN.

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