Favicon File Existence
Checks for the existence of a favicon image that is formatted correctly.
A favicon is a special image container with one or more icons inside of it that can be used by the web browser to display the website's icon in the address bar, open tabs, or bookmarks. Traditionally, this file was named "favicon.ico" and was stored at the root level of the website. In modern browsers you can specific a different location in the <head> section of the webpage, but legacy browsers and non-browser software may still look for it in the traditional location, so we recommend placing it here.
You can read more about the favicon file format on Wikipedia.