The history of cascading styles sheets is not too long. As we know that CSS (Cascading Style Sheet) is used in HTML and HTML is not too old. In the early days of HTML there was no CSS or Cascading Style Sheet available. All the browsers had their own internal CSS to show HTML pages. So the pages were styled by every browser with its own internal CSS (Cascading Style Sheet) and it was really a bit worried for the developers, because the developers do not have any control over the page styling.
In 1996 CSS 1.0 was released which gave the authors to style the HTML elements. It was really helpful for the developers to develop HTML pages as they like them to be shown in the browsers. After the release of CSS 1.0 the newer version named CSS 2.0 was released in 1998. These were the times when the web browsers development reached a very important phase. In the early days of browser development they did not focus on interoperability or standards. Some of them rather many of them had proprietary features and elements. That is why the CSS 2.0 was constantly updated even it was released. The CSS or Cascading Style Sheet specifications were not as formal as they are now we have. That was the reason that no browser fully implements the CSS 2.0 specifications till now.
These issues of CSS (Cascading Style Sheet) indorsed the W3C to revise the 2.0 specifications. It was pushed by the adoption by the browsers and the focus on web standards within the development community. Then CSS 2.1 was issued as a candidate recommendation. It is the CSS people generally talk about when they use the term CSS. It was released in 2004. So the support for web standards grow up with the development of CSS 2.1 and browser manufacturers finally get serious by implementing it seriously. In CSS 2.1 we have got fully implementation of the standard for most of the modern browsers. W3C pulled the CSS 2.1 specifications to working draft in 2005 because of the uneven implementation. Then after some developments in 2007 it was returned to candidate recommendation. It was become the fully proposed recommendation in 2011. So it took about 6 years to fully publish the CSS 2.1 specifications. It was the pace of web that made many of the specs outdated when they were published. The web application development and mobile web development were the technologies which demanded new needs in the CSS.
As the CSS modularization is keep on going the CSS3 term is introduced. You should be aware that the CSS3 is not a single specification. It is because over a very long time the development process in CSS was going on so it makes it a very large file that is very difficult to be hold as a document. Now CSS is published as a series of modularized specifications. It means that every section is in its own section like the selectors are now in their own specifications. This gave W3C the ability to prioritize specs and to react rapidly to the new needs in the field of CSS. So that was a brief history of cascading style sheets for now.