HTML5 `<link rel="shortcut icon" />`
From the same WHATWG document:
For historical reasons, the icon keyword may be preceded by the keyword "shortcut". If the "shortcut" keyword is present, it must be come immediately before the icon keyword and the two keywords must be separated by only a single U+0020 SPACE character.
Update: According to this page https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types
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.
Also, I don't see any references of shortcut in the W3Schools documentation, so I think it's better to leave it out. https://www.w3schools.com/tags/att_link_rel.asp
Although it is still in its experimental stage (because HTML5 dev is ongoing), the w3.org HTML5 Markup Validator is a good online tool you can check out. I used it extensively about 6 months ago and it always gave back accurate HTML5 syntax reports.
I have tested this code on Google Chrome, Mozilla Firefox, Internet explorer, and Microsoft Edge, and the only browser that didn't work was Edge. When add Favorite from a page made on harddrive ex c:\temp\test.html
<html>
<head>
<meta http-equiv="expires" content="0"><!-- Never caches the page -->
<link id="favicon" rel="shortcut icon" href="icon_32.ico">
</head>
<body>
...
</body>
</html>