Favicons are the ugly step-child of the web, a leftover from early Internet Explorer days and a bit awkward to handle as they don’t use web technologies you know and love, but rather fall back to the Windows .ico file format. However, it’s an important part of your web site, and it’s literally the first thing a users sees when looking at the URL bar in the browser. The favicon is also used in various other places, like tabs and bookmarks.

Interesting reading on making sure your Favicon for your site is Retina-ready..