It seems like the intent here was that someone like Twitter would provide a blue bird icon in SVG, and then specify that it is also suitable for use as a mask by providing the "mask" attribute. That icon could be used both for normal favicons and for this mask. Instead Twitter provided a black bird.
> You can set the icon that the user sees when they pin your site by providing a vector image. Use 100% black for all vectors with a transparent background in SVG format
Interesting, since their proposal says: "A simple author opt-in saying that an icon is suitable would help UAs pick the right icon to download and, unlike the sizes and type attributes, there's no need for a complicated attribute value microsyntax."
Mandating that it also has to be black seems like it would eliminate most existing favicons.
Especially since it would have been easy for the browser to derive an all-black mask from any image. Then sites only have to do extra work in order to support the new functionality, as opposed to what we see here where they have to do extra work in order not to break.
