In <link rel="icon">, is the sizes attribute required or optional?

Question

I note that in the WHAT-WG section on:

link rel="icon"

Every icon referenced includes a sizes attribute:

<link rel=icon href=favicon.png sizes="16x16" type="image/png">
<link rel=icon href=windows.ico sizes="32x32 48x48" type="image/vnd.microsoft.icon">
<link rel=icon href=mac.icns sizes="128x128 512x512 8192x8192 32768x32768">
<link rel=icon href=iphone.png sizes="57x57" type="image/png">
<link rel=icon href=gnome.svg sizes="any" type="image/svg+xml">
<link rel=stylesheet href=lsforums.css>

Source: https://html.spec.whatwg.org/dev/links.html#rel-icon

But, to my knowledge, the sizes attribute has never been required.

I'm trying to find an authoritative source confirming the sizes attribute is optional. I cannot.

In <link rel="icon">, is the sizes attribute required or optional?

I am specifically looking for an authoritative source - thanks.

Answer 1

I finally found what I was looking for. I've added emphasis to the quote below:

The sizes attribute gives the sizes of icons for visual media. Its value, if present, is merely advisory. If specified, the attribute [...] The attribute must only be specified on link elements that have a rel attribute that specifies the icon keyword or the apple-touch-icon keyword.

Source: https://html.spec.whatwg.org/dev/semantics.html#attr-link-sizes