The CSS spec does not mention it, but you can mimic C-style and/or unix-style line comments in CSS files (with some caveats). Others have written about them before (see in particular, SitePoint’s Web Foundations post covering CSS Comments). The present post examines them in further detail.
[Note: Thanks to Louis Lazaris and Web Tools Weekly, this post was originally published on October 21, 2015, at http://www.sitepoint.com/pseudo-comments-in-css-or-how-browsers-parse-styles/. It is actually a follow-up (slightly modified) to my earlier post on CSS
line rule comments. ]