We still asking ourselves: should we use rel="shortcut icon" or rel="icon" or both for our favicon?

Example code

<link rel="shortcut icon" href="/favicon.ico" type="image/x-icon">
<link rel="icon" href="/favicon.ico" type="image/x-icon">

Solution

Avoid rel="shortcut icon" and use only rel="icon".
According to MDN and rel attribute documentation:

The shortcut link type is often seen before icon, but this link type is non-conforming, ignored and web authors must not use it anymore.