The Content Documents specification uses an auto-updating reference to the latest W3C version of SVG so that changes can be immediately reflected in validation. Currently this is SVG 2.

Note that part of this change is that the previous restrictions on the use of animation elements and events is removed.

Reference: https://w3c.github.io/publ-epub-revision/epub32/spec/epub-contentdocs.html#sec-overview-relations-svg