Without Javascript they are unknown to the browser. So the browsers fallback to treating them like a block element (so, a div). It's not because custom element

Without Javascript they are unknown to the browser. So the browsers fallback to treating them like a block element (so, a div). It's not because custom elements "work without Javascript". It's because browsers are trying to work even in the presence of errors and invalid/unknown markup. Javascript is required for custom elements.