Single or Double Colon for CSS Pseudo-elements: Which Notation Should Developers Use?

Using Single or Double Colon Notation for Pseudo-Elements: A Decision for Developers

The CSS single-colon (: pseudo-element notation has been supported for backwards compatibility by modern browsers, while Internet Explorer 7 and 8 still utilize the double-colon (::) notation. This raises the question for developers: which notation should be used?

Some argue that utilizing both notations in combination via a comma can cater to both IE8 users and modern browsers. However, this approach is inadvisable. According to CSS 2.1 standards, if a user agent cannot parse a selector, it must ignore the entire rule block. Therefore, a CSS 2.1 compliant user agent will disregard the entire rule for a selector using both notations.

For improved compatibility with IE8, using both notations separately is an option:

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

However, this approach can be verbose and unnecessary.

For now, sticking with the single-colon notation is recommended as it maintains backwards compatibility with modern browsers while accommodating the declining market share of IE8. Once IE8's usage becomes negligible, a find-and-replace solution in the codebase can easily transition to the double-colon notation, offering greater support for pseudo-element styling across all browsers.

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