Recommended fragment identifiers for media overlays

[mattgarrish]

The previous requirement was that fragments in textref attributes and text element's src attributes be conformant to xpointer shorthand, but this syntax is incompatible with current XHTML (since IDs no longer have to be NCNames). I don't believe we've enforced this, though.

The new recommendation is that the fragment reference a target element (# followed by string without spaces) or be an SVG fragment identifier. Anything else should be a warning.

See https://github.com/w3c/epub-specs/issues/1586