You seem to know about the pros of protocol-relative URIs.
Cons might be:
- The links probably won’t work in a non-HTTP or non-HTTPS context (e.g., if your content is part of an email, or if your HTML page is opened locally)
- Some older browsers don’t support protocol-relative URIs (e.g., Internet Explorer 6)
- Some older browsers fetch protocol-relative linked resources twice, via HTTP and HTTPS (e.g., Internet Explorer 7+8)
Of course some resource should always be delivered via HTTPS, so don’t use protocol-relative URIs for these, enforce HTTPS instead.
And you should probably not use protocol-relative URIs for things like rel
-canonical
.