And there's no need, because there are multiple ways in HTML they could already express this without violating HTML. Here are 2 ways that work, and then the way they've done it which is in violation:
Possible method 1: Data attributes. Any time you as an author want to add additional data to an element, you are free to invent any custom attribute names you want, so long as they begin with "data-".
Possible method 2: Custom elements. Any time you as an author want to add a custom element to HTML, you are free to invent any tag name so long as it follows a simple naming criteria (essentially [a-z]+-[a-z]+) and if you invent any custom element, you are allowed to define whatever attributes on it you like.
Invalid method chosen: To pollute standard elements with invalid attribute names in violation of the HTML standard.