::before vs :before

Note the double-colon ::before versus the single-colon :before. Which one is correct?

Technically, the correct answer is ::before. But that doesn't mean you should automatically use it.

The situation is that:

• double-colon selectors are pseudo-elements.
• single-colon selectors are pseudo-selectors.

::before is definitely a pseudo-element, so it should use the double colon.

The distinction between a pseudo-element and pseudo-selector is already confusing. Fortunately, ::after and ::before are fairly straightforward. They literally add something new to the page, an element.

But something like ::first-letter is also a pseudo-element. The way I reason that out in my brain is that it's selecting a part of something in which there is no existing HTML element for. There is no <span> around that first letter you're targeting, so that first letter is almost like a new element you're adding on the page. That differs from pseudo-selectors which are selecting things that already exist, like the :nth-child(2) or whatever.

Even though ::before is a pseudo-element and a double-colon is the correct way to use pseudo-elements, should you?

There is an argument that perhaps you should use :before, which goes like this:

1. Internet Explorer 8 and below only supported :before, not ::before
2. All modern browsers support it both ways, since tons of sites use :before and browsers really value backwards compatibility.
3. Hey it's one less character as a bonus.

I've heard people say that they have a CSS linter that requires (or automates) them to be single-colon. Personally, I'm OK with people doing that. Seems fine. I'd value consistency over which way you choose to go.

On the flip side, there's an argument for going with ::before that goes like this:

1. Single-colon pseudo-elements were a mistake. There will never be any more pseudo-elements with a single-colon.
2. If you have the distinction straight in your mind, might as well train your fingers to do it right.
3. This is already confusing enough, so let's just follow the correctly specced way.

I've got my linter set up to force me to do double-colons. I don't support Internet Explorer 8 anyway and it feels good to be doing things the "right" way.

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