That's... that's what HTML5 means. It's not just "HTML", it includes all of the DOM and APIs that you use through JavaScript to make any of it mean anything. That the <audio> element is reskinnable with CSS in this way is part of the HTML5 standard.
And it's rather much splitting hairs. HTML isn't very useful without CSS, CSS isn't useful at all without HTML, and JS is the only choice of languages for interactivity in HTML. "Can we please stop using 'car' to mean 'car'+'engine'+'tires'".
No, because HTML4 really is a completely different beast. The syntax is 99% the same, the available elements are backwards compatible (though the recommended usage is almost completely different) and HTML4 did not include anything about DOM or APIs.
And it's rather much splitting hairs. HTML isn't very useful without CSS, CSS isn't useful at all without HTML, and JS is the only choice of languages for interactivity in HTML. "Can we please stop using 'car' to mean 'car'+'engine'+'tires'".