The release of OS El Capitan brought us Safari 9, which gave web developers a slew of new features to play with. One of my favorite new features is the ability to define an icon for pinned tabs. Implementing these icons is a snap. All you need is an SVG icon (16px square works best) and a link tag in the head of your document to include it:

<link rel="mask-icon" href="icon.svg" color="#322f31">

While working on our company website I ran into a situation where the pinned tab got stuck on the default letter in a rounded square. No amount of cache clearing had any effect. However, one terminal command saved the day:

rm ~/Library/Safari/Template\ Icons/*