KSES: Allow all Custom Data Attributes

[dmsnell]

Trac ticket: Core-61052

Allow spec-compliant data-attributes in wp_kses_attr_check().

In this patch the set of allowable data attribute names is expanded to permit any of the spec-compliant attributes starting with data-. There are complications in the term "spec-compliant" because custom data attributes must also follow some additional constraints which aren't applied here, but those constraints aren't upheld in a browser either.

These are recommendations. If these naming recommendations are not followed, no errors will occur. The attributes will still be matched using CSS attribute selectors, with the attribute being case insensitive and any attribute value being case-sensitive. Attributes not conforming to these three recommendations will also still be recognized by the JavaScript HTMLElement.dataset property and user-agents will include the attribute in the DOMStringMap containing all the custom data attributes for an HTMLElement.

https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/data-*

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