Are relative links valid in <link rel="alternate" hreflang="..." /> tags?

Question

Google's use of hreflang is documented here and all the examples have absolute links but there's no mention of absolute links being required or not. I found one post on a Google group saying they are required to be absolute but it doesn't seem authoritative and there's no reference. On the other hand W3C has this example:

<link rel=alternate type=application/pdf hreflang=fr href=manual-fr> 

Can anyone settle this?

Answer 1

Google says clearly in this April 2013 post (under "Mistake 2") that they consider the link tag to accept relative as well as absolute links.

Answer 2

The other answer (from 2015) is wrong.

Here's what the spec says today:

Alternate URLs must be fully-qualified, including the transport method (http/https), so: https://example.com/foo, not //example.com/foo or /foo

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You can also test your site using https://technicalseo.com/tools/hreflang/, it'll clearly tell you relative urls aren't supported.

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I believe alternate is the keyword here, standard <link> can use relative paths, but rel=alternate links must use absolute urls.