事实上，随着新模块的添加，CSS3在这个词的意义上可能永远不会达到“最终”状态。这是因为从3级开始，CSS本身已经被模块化了，这样每个模块可以独立于其他模块开发(尽管相关的模块可以一起开发)。这不仅允许现有模块独立调平，还可以随时创建新的模块，或者定义全新的功能集，或者扩展现有的CSS2.1功能。

基于CSS2.1规范现有部分的模块从第3层开始，而CSS的全新模块从第1级开始。现在，虽然“4级”似乎意味着有一个CSS4来了，就像“3级”经常用来指CSS3一样，CSSWG的成员之一就是published a blog post talking about the term “CSS4”说不是这样的：

THERE IS NO SUCH THING AS CSS4

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”.

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.

由于每个模块都是独立开发的，所以截至2012年，只有某些模块已达到或超过候选建议(CR)阶段。值得注意的包括：

> Media Queries
> Namespaces
> Colors
> Selectors
> Backgrounds and Borders
> Image Values and Replaced Content
> Multi-column Layout
> Values and Units

几十个其他模块中的大部分仍在草稿中，可能需要几个月或几年才能达到CR，PR或REC。如上所述，更多的是被添加到所有的时间，这些都将按照自己的步调。

有关CSS开发状况的信息，请参阅以下页面：

> CSS Current Status – W3C. Standards规定的规格已经被标准化为W3C建议书。这由W3C维护;然而，规范本身由CSS工作组维护，他们直接参与CSS的开发。
> CSS Current Work.这里有一个规格表，您可以一目了然地了解每个模块在开发中的成熟程度。此页面由CSS工作组维护。
> W3C CSS WG Note.本页面描述了CSS标准的开发过程。它提及术语“级别”，用于描述CSS标准的每个修订，而不是术语“版本”：

2. CSS Levels

Cascading Style Sheets does not have versions in the traditional sense; instead it has levels. Each level of CSS builds on the prevIoUs,refining definitions and adding features. The feature set of each higher level is a superset of any lower level,and the behavior allowed for a given feature in a higher level is a subset of that allowed in the lower levels. A user agent conforming to a higher level of CSS is thus also conformant to all lower levels.

另外，考虑到CSS的这种模块化以及某些模块的完成和标准化，这些模块的第4层已经开始了工作，例如Backgrounds and Borders和Selectors.但是，由于这些模块刚刚启动，所以不要指望供应商开始支持这些模块至少一年。如上所述，虽然这些模块正在进行到4级，但它们没有被正式定义或版本化为“CSS4”。