Custom protocol reported as invalid

[hubertgajewski]

Originally reported to HTML validator validator/validator#1949.

Steps to reproduce:

• Validate this code:
  .foo { background: url("foo://example.com/image.svg"); }

Result:
Validation failed (Value Error : background url("foo://example.com/image.svg") is an incorrect URL url("foo://example.com/image.svg")

Expected:
Validation passed.

I did not find in HTML/CSS specification any rule which suggests that I can't use custom protocol.