favicon.ico is Microsoft ico file. Some browser support png and git format. Its size 16x16 or 32x32 pixel and 8-bit or 24-bit in color depth.
Browser searched for /favicon.ico in the root directory of website. Or as W3C HTML recommend use in <head> tag:
<link rel="shortcut icon" href="http://example.com/favicon.ico" /> <link rel="icon" type="image/png" href="http://example.com/image.png"> <link rel="icon" type="image/gif" href="http://example.com/image.gif">
If more than one Web page shares the same shortcut icon, the shortcut icon is downloaded only once.
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