A favicon is a 16x16 pixel icon, sometimes packed with a 32x32 pixel icon (and rarely a 48x48 pixel or larger icon) into a file customarily named favicon.ico. Browsers display this icon on tabs open to their website. See the date-on-shield on a Google Calendar tab, or the rainbow-colored suitcase on eBay's tab.
There are dozens of online free favicon generators that pack the multiple images into an appropriate file, easily enough for a novice to use.
Has any thought been given to accommodating user-provided favicons?