XML does let you define tags via DTDs and schemas, but HTML Custom Elements let you _implement_ new tags, so that they do something. The point isn't really to

XML does let you define tags via DTDs and schemas, but HTML Custom Elements let you _implement_ new tags, so that they do something. The point isn't really to create new <div> or <span> tags, but new <img>, <a>, <input> and <select> tags, and of course, much more complex ones on top of those.