The WHATWG document for HTML5 says that the rel attribute must contain values that are space-separated, and then it gives a table of allowed values.
The attribute's value must be a set of space-separated tokens. The allowed keywords and their meanings...
The list of allowed keywords for the link element does not include shortcut, but it does include icon. So I'm looking at the all-too-well-known tag
<link rel="shortcut icon" href="/favicon.ico" />
and wondering if it is HTML5-compliant. Should I remove the keyword shortcut from this tag throughout my Website?
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.