One of the things that make front-end web development interesting, is that things change rapidly. There is always something new to learn and the web community is always figuring out better, faster, and more effective ways of solving problems.
For example, three years before writing this edition of the book responsive images (srcset and the picture element that are detailed in Chapter 3, Fluid Layouts and Responsive Images) simply didn't exist. Back then, we had to use clever third party workarounds to serve up more appropriate images to different viewport sizes. Now that common need has been rationalized into a W3C standard we can all now use and enjoy.
Similarly, not long ago, Flexbox was just a twinkle in a specification writer's eyes. Even when the specification evolved it was still difficult to implement until Andrey Sitnik and those clever folks at Evil Martians (https://evilmartians.com/) created Autoprefixer and we are subsequently able to use it cross-browser with relative ease.
The future holds yet more exciting capabilities for us to understand and implement. We've already mentioned Service Workers in Chapter 4, HTML5 for Responsive Web Designs, for example (http://www.w3.org/TR/service-workers/); a better way to create offline capable web-based applications.
There is also 'Web Components' a collection of standards made up of Shadow DOM (http://w3c.github.io/webcomponents/spec/shadow/), Custom Elements (http://w3c.github.io/webcomponents/spec/custom/) and HTML Imports (http://w3c.github.io/webcomponents/spec/imports/) that will allow us to create entirely bespoke and re-usable components.
Then there are the other forthcoming enhancements such as CSS Level 4 Selectors (http://dev.w3.org/csswg/selectors-4/) and CSS Level 4 Media Queries, which we covered in some detail in Chapter 2, Media Queries – Supporting Differing Viewports.
Finally, another big change looming on the horizon is HTTP2. It promises to make many of our current best practices, bad practices. For a good in-depth primer I'd suggest reading http2 explained by Daniel Stenberg (it's a free PDF). Alternatively, for a lighter summary, read Matt Wilcox's excellent post, HTTP2 for front-end web developers (https://mattwilcox.net/web-development/http2-for-front-end-web-developers).