Element: guarantee uniqueness

[tidoust]

As we're adding HTML Ruby Extensions, which monkey patches the HTML spec, I realize that we do not guarantee that elements will appear only once in elements extracts, (see Elements extracts guarantees). Should we?

I note that's easier said than done. On top of those introduced by HTML Ruby Extensions, there are duplicates already:

• elements in SVG11 and SVG2. Guarantee could be: "unique when considering series"
• elements in svg-animations and SVG11. We could apply the same superseding rules as for CSS.
• the filter element in filter-effects-1 and SVG11/SVG2. Idem.
• elements in epub-33 and HTML. Would require a new rule but namespaces are different, I think.

[dontcallmedom]

SVG and HTML also have duplicate elements (at least a & title off the top of my head), which have pretty different constraints/data models.