The comment for Element.tagName currently reads "Returns the HTML-uppercased qualified name." That's true of HTMLElement.tagName specifically, but not of Element.tagName generally (e.g., neither SVGElement.tagName nor MathMLElement.tagName is uppercase).
The comment for
Element.tagNamecurrently reads "Returns the HTML-uppercased qualified name." That's true ofHTMLElement.tagNamespecifically, but not ofElement.tagNamegenerally (e.g., neitherSVGElement.tagNamenorMathMLElement.tagNameis uppercase).