Why can you only use defer when src is specified?

Question

When validating code that uses the <script> tag I get the following warning on validator.w3.org

Element script must not have attribute defer unless attribute src is also specified.

I don't understand the need for this, could someone explain it?

Answer 1

The point of defer is to say "You can keep on parsing the HTML before waiting for the script to arrive".

If the script is right there in the HTML, then you can't (since the script is in the way in the source code) and it would be pointless (since there is no external dependancy that would block parsing).