Is this link valid?
Yes, it is.
You can find it in RFC 1808 - Relative Uniform Resource Locators:
Within an object with a well-defined base URL of
the relative URLs would be resolved as follows:
5.1. Normal Examples
RFC 3986 - Uniform Resource Identifier (URI): Generic Syntax restates the same, and adds more details, including the grammar:
relative-ref = relative-part [ "?" query ] [ "#" fragment ] relative-part = "//" authority path-abempty / path-absolute / path-noscheme / path-empty #; zero characters
Now, that is not to say all browsers implement it according to the standard, but it looks like this should be safe.