The custom element specification (section 4.13) deals mostly with the registration and usage details, but the general extensibility section earlier on covers the basic use case: https://html.spec.whatwg.org/multipage/infrastructure.html#e...

"User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them."