Do you know why there is no CSS4 after CSS3?

To put it simply, the CSS specification has been split into multiple modules (module) to be upgraded individually, starting with CSS3, or the new requirements as a new module for the project and standardized. So in the future there will be no more CSS4, CSS5 this so-called large version of the change, some just CSS a module level of the jump.

Before the introduction of modularity

According to the CSS Working Group, there is no version concept in CSS history, and some are just "level" concepts. For example, CSS3 is actually CSS level 3,CSS2 is CSS level 2, and CSS Level 1 is of course CSS1. Each level is based on one level above.

As we all know, CSS1 is already obsolete. CSS2 In fact is basically obsolete. However, CSS1, CSS2 (and CSS2.1) were a chatty norm at the time. Moreover, the CSS2 has not yet defined the CR phase in the process of standardization when it becomes the final standard.

Later, the development of the standard-setting process, requiring each specification to go through the following five stages:

1. Draft work

(Wd,working Draft)

2. Final draft work

(Lc/lcwd,last call working Draft)

3. Candidate recommendations

(Cr,candidate recommendation)

4. Proposed recommendation

(pr,proposed recommendation)

5. Recommended standards

(rec,recommendation)

Since the original CSS2 did not pass through the CR stage, so there are many problems. CSS working groups are being bothered by an endless "bug fix". As a result, they decided to make a revision to CSS2, which is CSS2.1 (CSS level 2 Revision 1).

In other words, CSS2.1 is actually just a revised version of CSS2, and there is no substantial change. Some of the content in the CSS2, the CSS Working Group considered not mature enough, and then removed from the CSS2.1. These deletions are considered to fall back into the CR phase of the process-equivalent to the need for "melted down". (Later, the content was basically revised and updated in CSS modules, and entered CSS Level 3.) ）

CSS2.1 and previous CSS specifications write everything in one document. As CSS features become more and more complex, the length of the CSS specification is growing. The PDF version of CSS2.1 has 430 pages. This has caused great inconvenience to errata and further upgrades. Because the progress of different parts of the document is not likely to be consistent.

The CSS Working Group then decided to take a modular route from CSS2.1 onwards. Is the need to upgrade the content independent into a module to split out, the new requirements are also in the form of a novel module project. Since then, CSS has entered level 3.

After the introduction of modularity

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

CSS Level 2 legacy modules: Selector, Color, Values and Units, backgrounds and borders are all modules that were removed from the original CSS specification. The naming of these modules starts with level 3, such as selectors level 3, CSS backgrounds and Borders level 3, because they all start with CSS Level 2.

New modules: Multi-column Layout, transitions, flexible box, transforms, and so on are the new modules that were added in the past, and no similar features existed in the previous CSS. So they will be named starting at level 1, such as CSS transitions level 1, CSS flexible Box Module level 1.

Of course, there are also CSS2.1 removed from the CSS2. As mentioned earlier, "the content in the CSS2 deleted by CSS2.1 is considered to fall back to the CR phase" and most of it will be "reincarnated" as CSS Level 3, and once they enter the CR phase, they will replace the previous corresponding content as the new standard.

OK, the specification of any of the above modules from WD to rec stage, either means that the new CSS module is born, or it means that the old CSS module has renewed its vitality!

Hopefully, the following diagram will illustrate the naming of CSS modules more visually:

The module will also have Level 4 or higher

CSS to Level 3 is no longer evolving to level 4 because of the "modular" approach. Because of this, CSS never appears CSS4 in theory. However, a module in CSS can go to level 4 or higher. For example, the CSS Color module starts the Level 4 upgrade Tour (HTTPS://WWW.W3.ORG/TR/CSS-COLOR-4/).

That's what CSS snapshot 2017 says:

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

No CSS Level 4. Standalone modules can go to Level 4 or higher, but CSS does not have this level of language anymore. ("CSS Level 3" is a concept that is only easy to distinguish from previous versions of Chatty.) ）