Do you know why CSS3 will not be followed by CSS4?

To put it simply, starting from CSS3, the CSS specification has been split into many modules (modules) for individual upgrades, or new requirements have been established and standardized as a new module. Therefore, there will no longer be so-called major version number changes such as CSS4 and CSS5 in the future, but only a certain module-level transition of CSS.

Before the introduction of modularity

According to the CSS working group, there is no concept of version in the history of CSS, only the concept of "level". For example, CSS3 is actually CSS Level 3, CSS2 is CSS Level 2, and CSS Level 1 is of course CSS1. Each level builds on the previous level.

Everyone knows that CSS1 has been obsolete a long time ago. CSS2 is actually basically obsolete. However, CSS1, CSS2 (and CSS2.1) were large and comprehensive specifications at the time. Moreover, when CSS2 became the final standard, the CR stage had not yet been defined in the W3C specification process.

Later, W3C further improved the specification development process, requiring each specification to go through the following five stages:

1. Working Draft

(WD, Working Draft)

2. Final working draft

（LC/LCWD, Last Call Working Draft）

3. Candidate recommendation

（CR, Candidate Recommendation）

4. Proposed recommendation

(PR, Proposed Recommendation)

5. Recommended standard

(REC, Recommendation)

Because the original CSS2 has not gone through the CR stage, so there are many problems. The CSS working group was overwhelmed with endless "bug fixes". As a result, they decided to make a revision of CSS2, which is CSS2.1 (CSS Level 2 Revision 1).

In other words, CSS2.1 is actually just a revised version of CSS2, with no substantial changes. Some content in CSS2 was considered immature by the CSS working group and was deleted from CSS2.1. These deleted contents are considered to be rolled back to the CR stage of the process - equivalent to the need for "rewinding". (Later, these contents were basically in the form of CSS modules, revised and supplemented, and entered CSS Level 3.)

CSS2.1 and previous CSS specifications wrote all the contents in one document. As CSS features become more and more complex, the CSS specification becomes longer and longer. The PDF version of CSS2.1 has 430 pages. This brings great inconvenience to errata and further upgrades. Because the upgrade progress of different parts of the document cannot be forced to be consistent.

So, the CSS working group decided to take the modular route starting from CSS2.1. That is to say, the content that needs to be upgraded is separated into independent modules, and new requirements are also established as new modules. Since then, CSS has entered Level 3.

After the introduction of modularity

After CSS adopts the modular route, there are three types of modules, and their naming methods are very noteworthy.

CSS Level 2 original modules: Selector, Color, Values ​​and Units, BackgroundS and Borders, etc. These are all modules separated from the original CSS specification. The naming of these modules will start from Level 3 from the beginning, such as Selectors Level 3, CSS Backgrounds and Borders Level 3, because they all start on the basis of CSS Level 2.

New modules: Multi-column Layout, Transitions, Flexible Box, Transforms, etc. are all newly added modules. Similar features did not exist in CSS before. Therefore, their naming will start from Level 1, such as CSS Transitions Level 1 and CSS Flexible Box Module Level 1.

Of course, there are also things that CSS2.1 removed from CSS2. As mentioned before, "the content in CSS2 that was deleted by CSS2.1 is considered to have fallen back to the CR stage", and most of the content will be "reincarnated" in the form of CSS Level 3. Once they enter the CR stage, they will It will replace the previous corresponding content and become the new standard.

OK, the advancement of the specification of any of the above modules from WD to the REC stage either means the birth of a new CSS module or the rebirth of an old CSS module!

I hope the following picture can explain the naming of CSS modules more intuitively:

##Modules will also have Level 4 or higher

CSS to Level 3 will not evolve to Level 4 because of the "modularization" strategy adopted. Because of this, CSS will theoretically never see CSS4. However, a certain module in CSS can reach Level 4 or higher. For example, CSS Color Module has begun its upgrade journey to Level 4 (https://www.w3.org/TR/css-color-4/).

This is what CSS snapshot 2017 said:

There is no CSS Level 4. Independent modules can reach level 4 or beyond, but CSS the language no longer has levels. (“CSS Level 3” as a term is used only to differentiate it from the previous monolithic versions.)

No CSS Level 4. Independent modules can reach Level 4 or higher, but the CSS language will no longer have this level. (“CSS Level 3” as a concept is just to distinguish it from the previous large and comprehensive version.)

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