While I was looking at the HTML source for my favorite website, I noticed that it's using
<link rel="shortcut icon" href="favicon.ico">
<link rel="apple-touch-icon image_src" href="apple-touch-icon.png">
Obviously the second link has something to do with Apple's iOS, but why isn't <link rel="shortcut icon" href="favicon.ico"> sufficient? What is special about iOS that it has a dedicated html link rel tag?