Can I use a relative URL in my link tag?
In general, yes; but if you wish to register for ICRAchecked, and your site is accessible with and without the www prefix to your domain, then no, you must use an absolute URL in the link tag's href attribute.
Why? Filters and the ICRA label tester will follow a relative URL in your link tag, for example, to "/labels.rdf" readily enough. Your site *is* labelled is you use a relative URL. However, the ICRAchecked system provides a database of the URLs of labels files that have been checked. It is therefore critical that the labels file is always referred to by the same URL. An example will explain this.
The example.com website is accessible at both http://example.com and http://www.example.com. The labels.rdf file is in the root directory and the link tag gives "/labels.rdf" as the value for the href attribute. The webmaster registered for ICRAchecked and gave http://example.com as the web address. The system followed the link and recorded the URL of the labels file as http://example.com/labels.rdf.
If a client visits http://www.example.com/labels.rdf and follows the link to the labels file, it will see its URL as http://www.example.com/labels.rdf and query the ICRAchecked database about _that_ URL. Since the URL in the database and the URL under test are different, the labels file will not be show as being in the ICRAchecked system.
To avoid this, the href attribute on the link tag should always be an absolute URL, such as http://www.example.com/labels.rdf. That way, the ICRAchecked database and the URL of the labels.rdf file will always match.