Media Queries Level 4
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 WD status with no support in browsers. Meanwhile the topic of this chapter, Media Queries Level 3, 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, while CSS Media Queries Level 4 enjoys a draft specification: http://dev.w3.org/csswg/mediaqueries-4/, not all the features it documents enjoy browser implementations. So in this section...