Is Element.tagName always uppercase?

Question

Reading at MDN about Element.tagName it states:

On HTML elements in DOM trees flagged as HTML documents, tagName returns the element name in the upper

Answer 1

tagName and nodeName comes in lowercase or case sensitive, in case page response headers is having Content-type: application/xhtml+xml; header or any xhtml related content type. e.g. document.documentElement.nodeName returns html Read this for more cases: https://www.w3.org/TR/REC-DOM-Level-1/level-one-core.html#ID-745549614