THERE IS NO SUCH THING AS CSS4

以前没有css4.将来也不会有。CSS4是不存在的。

在css2.1发布之后的所有东西都叫做css3。

CSS就是这门语言的最新术语，从今往后就用css表示这门技术，最好放弃后面的数字吧。

“但是”，你可能反对，“我看见过诸如CSS4 Images或者Selector 4！难道他们不是CSS4吗？”。

不是。就像我刚才说的，没有CSS4。^_^

当我们完成CSS2.1的时候，我们（CSSWG）意识到大版本不好。很难维护和开发速度慢。

相反，我们决定把CSS语言分解到一堆独立的模块里面。每个模块能独立的升级，并且只包含最小的特性，这样一个大的特性很难被一个小的棘手的特性拖慢。（只影响这个模块的升级，其他的可以独立升级）。

如果模块扩展了CSS2.1的功能，那么这些模块是从level3开始升级的的。其他新的模块都从level1开始（比如，flexbox）.但是一个模块所在的level与它所在的CSS版本没有任何关联。无论他们在哪个level都叫做CSS3或CSS。

那么，现在你知道了。没有CSS4这回事儿。只有CSS,并且每个模块都能独立升级。

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

The term "CSS3" refers to everything published after CSS 2.1. 
CSS is on its last version as a language as a whole, so it would be appropriate to just drop the number entirely and refer to everything from now on as just "CSS".

"But", you might object, "I saw things like CSS4 Images, or Selectors 4! Aren't they part of CSS4?".

No. As I just said, there is no CSS4. ^_^

While trying to finish CSS 2.1, we (the CSSWG) realized that big monolithic "versions" weren't any good. They were difficult to maintain, and slow to develop.

Instead, we decided to split up the CSS language into a bunch of independent modules. Each module can level up independently, and contains only a smallish set of features, so it's harder for a large set of features to be slowed down by a single stubborn feature.

Some of our modules start out at level 3, if they extend something from CSS2.1. Others start out at level 1, if they're something new (for example, Flexbox). However, the level that a module is at has no correlation with what version of CSS it's in. They're all CSS3 (or just CSS), regardless of what level they're at.

Our URLs don't help the matter much, of course. We use URLs with things like css4-backgrounds in them, which totally sounds like CSS4. On behalf of the CSSWG, I apologize for the confusion. All that means is that it's CSS Background & Borders Level 4, which is implicitly part of CSS3 (or just CSS).

So, now you know. There's no such thing as CSS4. There is only CSS, and each module can level up independently. Pass it on!