A favicon, or “favorite icon,” is small icon used to show your website and reinforce your brand in a browser.
Design: Since they are small, favicons must adhere to a clean design style that makes the brand clearly identifiable.
Size: Favicons should be 16x16 pixels. You can design them in larger dimensions as long as they can be exported cleanly to the proper size.
The default shape of a favicon is square, and can be saved in .jpg, .gif, .ico and .png formats. If your favicon isn’t square, you’ll need save it as a .png to ensure a transparent background (otherwise it will be white).
Where applicable, the ACS Phoenix favicon 
should be used.