The proof, as they say, is in the pudding: https://codepen.io/WickyNilliams/pen/dydoNez https://codepen.io/WickyNilliams/pen/dydoNez It's been supported for ye

The proof, as they say, is in the pudding: https://codepen.io/WickyNilliams/pen/dydoNez https://codepen.io/WickyNilliams/pen/dydoNez It's been supported for years AFAIK. As long as you extend HTMLElement, and not some other element, everything will work as expected