HTML is designed with extensibility in mind for data that should be associated with a particular element but need not have any defined meaning. data-* attributes allow us to store extra information on standard, semantic HTML elements without other hacks such as non-standard attributes, or extra properties on DOM....id="electric-cars" data-columns="3" data-index-number="12314" data-parent="cars">...addition, search crawlers may not index data attributes' values. See...