Tab Atkins in 2012:

There has never been a CSS4. There will never be a CSS4. CSS4 is not a thing that exists.

Rachel Andrew in 2016:

While referring to all new CSS as CSS3 worked for a short time, it doesn’t reflect the reality of where CSS is today. If you read something about CSS3 Selectors, then what is actually being described is something that is part of the CSS Selectors Level 3 specification. In fact CSS Selectors is one of the specifications that is marked as completed and a Recommendation. The CSS Working Group is now working on Selectors Level 4 with new proposed features plus the selectors that were part of Level 3 (and CSS 1 and 2). It’s not CSS4, but Level 4 of a single specification. One small part of CSS.

Jen Simmons in 2018:

Many people are waiting for CSS4 to come out. Where is it? When will it arrive? The answer is never. CSS4 will never happen. It’s not actually a thing.

So CSS3 was a unique one-off opportunity. Rather than one big spec, break them into parts and start them all off at “Level 3” but then let them evolve separately. That was very on purpose, so things could move quicker independently.

The problem? It was almost too effective. CSS3, and perhaps to a larger degree, “HTML5”, became (almost) household names. It was so successful, it’s leaving us wanting to pull that lever again. It was successful on a ton of levels:

It pushed browser technology forward, particularly on technologies that had been stale for too long.

It got site owners to think, “hey maybe it’s a good time to update our website.“

It got educators to think, “hey maybe it’s a good time to update our curriculums.“

It was good for the web overall, good for websites taking advantage of it, and there was money to be made along the way. I bet it would be staggering to see how much money was made in courses and conferences waving the CSS3 flag.

Peter-Paul Koch in 2020:

I am proposing that we web developers, supported by the W3C CSS WG, start saying “CSS4 is here!” and excitedly chatter about how it will hit the market any moment now and transform the practice of CSS. Of course “CSS4” has no technical meaning whatsoever. All current CSS specifications have their own specific versions ranging from 1 to 4, but CSS as a whole does not have a version, and it doesn’t need one, either. Regardless of what we say or do, CSS 4 will not hit the market and will not transform anything. It also does not describe any technical reality. Then why do it? For the marketing effect.

I think he’s probably right. If we all got together on it, it could have a similar good-for-everybody bang the way CSS3 did.

If it’s going to happen, what will give it momentum is if there is a single clear message about what it is. CSS3 was like:

border-radius

gradients

animations and transitions

transforms

box-shadow

Oh gosh, it’s hard to remember now. But at the time it was a pretty clear set of things that represented what there was to learn and they were all fairly exciting.

What would we put under the CSS4 flag?

Flexbox? (Too old?)

Grid

Everything new with color (like this and this)

Independent transforms

Variable fonts

Offset paths

Let’s get nesting done!

Houdini stuff? (Not ready enough?)

Shadow DOM selectors?

Lemme just say I will personally spearhead this thing if container queries can get done and we make that a part of it.

What else? Wanna refute anything on my list?