Enhancement: warn when <desc> element is missing

[ajeanmahoney]

The SVG element allows an author to provide a description of the artwork, and the contents of the element can be read by webpage readers, which makes the artwork more accessible to people who are visually impaired.

It would be nice if svgcheck warned if the element is missing or empty.

[duerst]

The description will be in a <title> or element (or distributed over several of these). I agree this should be checked/warned. However, it's important to realize that a mechanical check is not enough, because tools cannot distinguish between a series of characters without meaning and a good description.