Style sheets have been in existence for over 35 years, with the original version of what we now know as CSS dating from the days of SGML in the 1980s.

We have come a long way since the original CSS standard was released in 1996—over the last few years, the standard for CSS4 has been developed, with new features such as :not or :matches pseudo-classes to better target elements, custom properties (or variables), and location-based links, such as local-link. Over the course of the next few pages, we're going to explore a few of these CSS4 features, and see how we can introduce support for them using current CSS3 equivalent code.

There is one small thing that we need to clear up first—CSS4…does not exist. What? I hear you say. Surely it must, I've seen plenty online about it! Yes, it is true: CSS4 as a standard does exist, but not as a single unique entity. Let me explain.

Previous iterations of CSS have been based around creating a single global standard, irrespective...