Comment by auxiliarymoose

Here is an alternative vanilla approach that uses a single-file/single-class-declaration custom element without Shadow DOM: https://jsfiddle.net/auxiliarymoose/tcgk1Ljv/98/

Usually I write a few helper functions to streamline working with elements, attributes, and registration of element + CSS. But even without those, I think this approach provides a good level of simplicity without introducing libraries or frameworks.