Regarding SPDX.
Though doing it with an HTML comment works:
<?xml version="1.0" encoding="UTF-8"?>
<!--
SPDX-FileCopyrightText: 2021 John Doe <[email protected]>
SPDX-License-Identifier: CC-BY-SA-4.0
-->
<svg version="1.2">
...
when editing the file in a GUI editor (e.g. Inkscape) however, the comment is removed after saving. Thus it is not a viable option.
Maybe these could be (ab-)used?:
- https://spdx.github.io/spdx-spec/v2-draft/snippet-information/#97-snippet-comments-on-license-field
- https://spdx.github.io/spdx-spec/v2-draft/snippet-information/#98-snippet-comments-on-license-field
NOTE
The REUSE tool auto-checks projects for SPDX/licensing conformity, but does not yet support SVG (nor XML specifically). It does support HTML, which is why the HTML comment thing would work, If editors would not strip it.
There is metadata in SVG, e.g. here for a description: https://www.w3.org/TR/SVGTiny12/metadata.html .
It also mentions RDF and IIRC SPDX has support for RDF (e.g. https://spdx.org/rdf/spdx-terms-v1.2/terms.html) - or a more current doc: D.4.6 License Expressions in RDF.
Inkscape does not remove the meta-data, even offers an editor for it.