I wrote this book to help you decide when to choose a circle over a rectangle or a line (elements), whether to make the circle large, small, red, green, or blue (attributes), and how to incorporate the circle with all the other circles and triangles and dots and lines and images and text in your design (principles).
Developing responsive sites and applications today requires flexible layouts. Modern CSS, including the flexible box layout module, provide the tools needed to build sites that adapt to the canvas in which their viewed. This book will walk you through the ins and outs of flexbox so you can use it in your projects today.
CSS is a relatively easy language to learn, but after awhile you wish it could do more. Sass effectively extends CSS so it can do more. It allows you to add features to CSS to make your code more powerful and easier to organize and maintain. The best part is, if you already know CSS, then you already know the basics of Sass and you can learn more advanced features as you want or need.