title: Is Element.tagName always uppercase?
body: <p>Reading at <a href="https://developer.mozilla.org/en-US/docs/Web/API/Element.tagName" rel="noreferrer">MDN about Element.tagName</a> it states:</p>
  
  <blockquote>
    <p>On HTML elements in DOM trees flagged as HTML documents, tagName returns the element name in the uppercase form.</p>
  </blockquote>
  
  <p>My question is: is this trustable? Does IE (old and modern) behave as expected? Is this likely to change? or is it better to always work with <code>el.tagName.toLowerCase()</code>?</p>