Safari's pinned mask-icon, as modified in OS X 10.11

[grazianoappbuilder]

Hi, I found in GitHub's page itself a html element with the "mask-icon" attribute that I didn't know about.
This tutorial explains that it was introduced by Apple for Safari's pinned mask icons. There's already a pull-request on this topic, but it's outdated, as Apple eventually changed the syntax.
Quoting 1st paragraph:

With the release of Mac OS X 10.11, Safari finally got pinned tabs. While all other browsers use a site’s favicon for the pinned tab, Apple deemed that “not esthetically pleasing enough” and created a new type of icon for it, which they call a “mask-icon”. By default, they’ll use the first letter of your domain if you don’t have such an image. You need a black SVG image, a hover color and some time to do a tiny bit of coding.

Could you please integrate it in your document? I am new user and I'm not skilled in GitHub (unfortunately). Thanks. Graziano