It's a digestible introduction, but in the real world you'll find that there's a lumbering iceberg underneath. Display, alignment, justification, relative parents, stacking contexts, flow, negative margins, flexbox, transform... all of these interact in... let's say interesting ways, and not consistently across browsers. I wish CSS positioning could be explained with a few GIFs, but moving outside the playground you…
And here's another good one that is specific to layouts: http://learnlayout.com/display.html