Only if you need to define (JS) behavior for them. If you don’t, any arbitrary name will do (and will be an instance of HTMLUnknownElement[1]). Using hyphenated

Only if you need to define (JS) behavior for them. If you don’t, any arbitrary name will do (and will be an instance of HTMLUnknownElement[1]). Using hyphenated names is still advised, to avoid name collisions with future standard elements. 1: https://developer.mozilla.org/en-US/docs/Web/API/HTMLUnknownElement https://developer.mozilla.org/en-US/docs/Web/API/HTMLUnknown...