[kbenson]

I imagine if it is optimized, using data attributes will be the most likely route. There's probably already benefits (and thus it may already be done) to cache which elements have specific data attributes set. Combining that with a lazy indexing of values of matching attribute selector rules on elements with matching data elements on first match may yield non-horrible performance.

[masklinn]

Using data attributes is even less likely to ever be optimised as data attributes exist specifically to store browser-opaque programmatic content.

[kbenson]

I imagine data attributes are already optimized in some respects (probably not as selectors though), since their intended usage is slightly different than other attributes. I only contend that if any type of attributes are likely to receive special treatment in the future (besides those already designated as so, such as id and class), data attributes may be a likely candidate. Then again, due to the lack of constraint on the value of the data tags, that would make future optimizations much harder (which may have been your point).

Maybe just using class-* as a namespace would be best, both from a point of clearly identifying intent and not conflicting with any current valid html attributes (AFAIK).