New Browser-tracking Favicon Hack Works Even When Flushing Caches or Incognito

> Browsers save the icons in a cache so they don't have to request them over and over. This cache isn't emptied when users clear their browser cache or cookies, or when they switch to a private browsing mode.

I wonder why all major browsers implemented it this way. Some kind of compromise with the default behavior of doing a GET /favicon.ico for every root page, even when no favicon is defined in the document?