<link rel="apple-touch-icon">

It is not in the HTML specification: it is a Safari extension, consulted when someone adds the page to the home screen of an iPhone or iPad. It became a de facto standard — other browsers and readers use it as the high-resolution icon when they find nothing better.

Without it, iOS crops a thumbnail of the page itself as the icon, with the result you would imagine. A single square PNG of 180 pixels is enough — the device resizes and rounds the corners on its own; the old list of sizes with sizes is archaeology.

<link rel="apple-touch-icon" href="/apple-touch-icon.png">