The fourth edition of Eric Meyer and Estelle Weyl's CSS: The Definitive Guide was recently released. The new book weighs in at 1,016 pages, which is up drastically from 447 in the third edition, which was up slightly from 436 in the second edition.
Despite the appearance of CSS needing more pages to capture more complicated concepts, Eric suggests that CSS is actually easier to grasp than ever before and its complexity has actually declined between editions:
But the core principles and mechanisms are no more complicated than they were a decade or even two decades ago. If anything, they’re easier to grasp now, because we don’t have to clutter our minds with float behaviors or inline layout just to try to lay out a page. Flexbox and Grid (chapters 12 and 13, by the way) make layout so much simpler than ever before, while simultaneously providing far more capability than ever before.
In short, yes, lots of new concepts have been introduced since 2007 when the third edition was released, but they're solving the need to use layout, err, tricks to make properties bend in ways they were never intended:
It’s still an apparent upward trend, but think about all the new features that have come out since the 3rd Edition, or are coming out right now: gradients, multiple backgrounds, sticky positioning, flexbox, Grid, blending, filters, transforms, animation, and media queries, among others. A lot of really substantial capabilities. They don’t make CSS more convoluted, but instead extend it into new territories.
Hear, hear! Onward and upward, no matter how many pages it takes.