Widget needs to validate on HTML 4 pages

[julietteculver]

The widget uses the HTML 5 data- attribute which does not validate in an HTML 4 validator.

From Sebastian:

I've been looking at this. The issue right now is that the widget uses HTML5 data-* attributes and Nomisma is xhtml 4 so there are validation issues. As in, Nomisma strives to get the green light from the W3 validator (see http://validator.w3.org/check?uri=http%3A%2F%2Fnomisma.org%2Fid%2Fathens
) and so can't currently mix RDFa and HTML5 constructs in the markup it emits.

The work around is to introduce @data-pelagios_id after load via DOM manipulation and then call the Pelagios script. But long-term, it seems a little unclear if data-* attributes are compatible with any XHTML variant in the absence of local modifications to the DTD so this might be a recurring problem for us. An option for recognizing markup along the lines of '' would be one way to avoid this issue.