Should I Use Single or Double Colon Notation For Pseudo-Elements

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.

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.

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 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.

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.

Does pseudo first-line need double colon syntax?

As MDN states:

In CSS 2, pseudo-elements were prefixed with a single colon character.
As pseudo-classes were also following the same convention, they were
indistinguishable. To solve this, CSS 2.1 changed the convention for
pseudo-elements. Now a pseudo-element is prefixed with two colon
characters, and a pseudo-class is still prefixed with a single colon.

As several browsers already implemented the CSS 2 version in a release
version, all browsers supporting the two-colon syntax also support the
old one-colon syntax.

If legacy browsers must be supported, :first-line is the only viable
choice; if not, ::first-line is preferred.

Further, as the W3 states:

This :: notation is introduced by the current document in order to
establish a discrimination between pseudo-classes and pseudo-elements.
For compatibility with existing style sheets, user agents must also
accept the previous one-colon notation for pseudo-elements introduced
in CSS levels 1 and 2 (namely, :first-line, :first-letter, :before and
:after). This compatibility is not allowed for the new pseudo-elements
introduced in this specification.

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 the difference between two double point and one in pseudo-selector

This is to differentiate between pseudo-classes (such as :hover, :focus, :active) and pseudo-elements (such as ::before, ::after, ::first-line).

This is introduced as part of CSS3, by the World Wide Web Consortium (the W3C), but because pseudo-elements were introduced prior to this syntactic differentiation browsers support both ::before and :before.

According to the reference at the MDN (Mozilla Developer Network):

Browser            |  Lowest Version    |    Support of
-------------------+--------------------+------------------
Internet Explorer  |  8.0               |   :pseudo-element
                   +--------------------+------------------
                   | 9.0                |   :pseudo-element
                   |                    |   ::pseudo-element
-------------------+--------------------+------------------
Firefox (Gecko)    | 1.0 (1.0)          |  :pseudo-element
                   +--------------------+------------------
                   | 1.0 (1.5)          |  :pseudo-element
                   |                    |  ::pseudo-element
-------------------+--------------------+------------------
Opera              | 4.0                |  :pseudo-element
                   +--------------------+------------------
                   | 7.0                |  :pseudo-element
                   |                    |  ::pseudo-element
-------------------+--------------------+------------------
Safari (WebKit)    | 1.0 (85)           |  :pseudo-element
                   |                    |  ::pseudo-element
-------------------+--------------------+------------------

Note, from the CSS Selectors Recommendation:

This :: notation is introduced by the current document in order to establish a discrimination between pseudo-classes and pseudo-elements.

For compatibility with existing style sheets, user agents must also accept the previous one-colon notation for pseudo-elements introduced in CSS levels 1 and 2 (namely, :first-line, :first-letter, :before and :after).

This compatibility is not allowed for the new pseudo-elements introduced in this specification.

References:

CSS 2.1 Recommendation.
Pseudo-elements.

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.

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 is the difference between :before and ::before?

This distinguishes pseudo elements from pseudo classes.

The difference between pseudo classes and pseudo elements is described at http://www.d.umn.edu/~lcarlson/csswork/selectors/pseudo_dif.html

Should I Use Single or Double Colon Notation For Pseudo-Elements