rel="shortcut icon": What It Means
rel="shortcut icon" is an old way of declaring a favicon that dates back to Internet Explorer. The "shortcut" keyword is non-standard and ignored by browsers (only "icon" is actually honoured), so a plain rel="icon" does exactly the same job. Existing tags still work, but there is no reason to write "shortcut icon" in new markup.
Where it came from
Early Internet Explorer would only recognise a favicon if the link said rel="shortcut icon". Other browsers never needed the "shortcut" part. The HTML specification defines the "icon" keyword, not "shortcut icon".
Browsers treat the rel value as a space-separated list of keywords and match the ones they know. In "shortcut icon" they find "icon" and use it, and quietly ignore "shortcut". So the tag works, but the extra word does nothing.
Legacy versus modern
<!-- legacy, still works but redundant -->
<link rel="shortcut icon" href="/favicon.ico" />
<!-- modern equivalent -->
<link rel="icon" href="/favicon.ico" sizes="any" />"shortcut icon" FAQ
Is shortcut icon deprecated?
The "shortcut" keyword was never part of the standard, so there's nothing formal to deprecate, but it's obsolete in practice. Modern guidance is to use rel="icon" alone.
Does rel="shortcut icon" still work?
Yes. Browsers match the "icon" keyword inside it and use the file, ignoring "shortcut". The favicon loads exactly as it would with a plain rel="icon".
What's the difference between "shortcut icon" and "icon"?
None in effect. "icon" is the standard keyword; "shortcut icon" is that same keyword with a redundant legacy word in front of it.