Does anyone know why this is? Or how to fix it? It isn't a huge deal, but I would like to get rid of the warnings, if possible. Thank you very much.
Custom attributes (ie. 'data-???') are apparently a relatively recent addition to HTML 5, so the validator may not yet be aware of them. If it is aware of them, it may be issuing a warning just to let you know of them in case you've made a mistake. In any case, the warnings are nothing to worry about here.