Apple Touch Icon
Checks that the "apple-touch-icon.png" is defined, located in the correct spot and is the right size.
One of the features that Apple introduced on the iPhone was the ability to make a shortcut to a website and have it appear as an icon along with all your other apps. They did this by looking for a special image named "apple-touch-icon.png" and stored at the root level of the website. Over time, software and devices other than the iPhone have started using this file. The recommended size of this image has increased as higher resolution screens have appeared, and it is now possible to place the file somewhere other than root. However, for legacy reasons we still recommended that it be placed in the original location. Here is the recommended code to add to the <head> section of a website.
<link rel="apple-touch-icon" sizes="180x180" href="/apple-touch-icon.png">
This test will check for the presence of a <link> tag with the appropriate size and then download the image to make sure it is the proper size and format.
For more information, please read the documentation on Apple's Developer Website.