Media Queries Level 5
Specifications at the W3C go through a ratification process, from Working Draft (WD) to Candidate Recommendation (CR) to Proposed Recommendation (PR) before finally arriving, many years later, at W3C Recommendation (REC). So modules at a greater maturity level than others are generally safer to use. For example, CSS Spatial Navigation Level 1 is in progress as I write this (http://www.w3.org/TR/css-nav-1/), at Working Draft status with no support in browsers. Meanwhile, the topic of this chapter until this point has been Media Queries Level 3, which is implemented in every modern browser.
If you have a spare day, you can knock yourself out reading all about the official explanation of the standards ratification process at http://www.w3.org/2005/10/Process-20051014/tr.
At the time of writing, whilst CSS Media Queries Level 5 enjoys a draft specification: https://www.w3.org/TR/mediaqueries-5, not all the features it documents enjoy browser implementations...
