Semantically correct way to add a copyright notice into a svg file?

I want to add a copyright notice in my svg files and it should be only "hidden" text and no watermark. This is no real protection, because if you open a svg file with a text editor you can edit everything and delete the copyright. But I think this would be a simple and great way to show, who has made the file and a possible chance to find unlicensed graphics if there is some hidden information and if you are looking for it you can easily find it.

My main question is: how should the copyright text be put into the file?

<title> element is for accessibility purposes, some user agents display the title element as a tooltip.
<desc> element generally improves accessibility and you should describe what a user would see.
ugly way: a text element with inline CSS to hide it. Don't even think about this! :)
 could be also a simple solution.
<metadata>: this would the best way but I did not find a detailed definition and which child elements could live inside. Also https://developer.mozilla.org/en-US/DOM/SVGMetadataElement gives a 404. Under https://www.w3.org/TR/SVG/struct.html#MetadataElement we can find more details. But is RDF really necessary?

I think a <metadata> element is the right place, but which child elements should be used and is just RDF the way to go?

Solution

I think the metadata element is the correct choice here. It has to contain XML, but it doesn’t have to be a RDF serialization (e.g., RDF/XML).

But I think it makes sense to use RDF here, because that’s exactly RDF’s job (providing metadata about resources, like SVG documents), and there is probably no other XML-based metadata language that has greater reach / better support.

A simple RDF statement (in RDF/XML) could look like this:

<metadata>
  <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:schema="http://schema.org/">
    <rdf:Description rdf:about="http://example.com/my-svg-file.svg">
      <schema:license rdf:resource="https://creativecommons.org/licenses/by-sa/4.0/"/>
    </rdf:Description>
  </rdf:RDF>
</metadata>

The about attribute takes an IRI as value; for a stand-alone SVG document, you could provide an empty value (= the base IRI of the document).

In this example I use the license property from Schema.org:

A license document that applies to this content, typically indicated by URL.

(The vocabulary Schema.org is supported by several big search engines.)