I've often just wanted a "simple SVG" subset with just basics, as you describe, omitting all the fancy stuff that tends to lead to surprises and incompatibilities. For example, as the format for a very simple icon editor.

Do you know if there is some formal way to 'declare' such a subset (something to do with XML/namespaces maybe?), or if there's any existing standard "simple SVG" format that someone has already defined? Obviously I could just informally stick to a subset by avoiding whatever features I don't want to use, but I think it would be useful to have a formally defined subset, and to be able to scan an SVG to detect if it's within the subset or not.