[d_k_f]

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."

[troupo]

Yes. It means they are not "custom elements". They are basically divs.

[xigoi]

Yes, they are basically divs, but with a semantic name. Which is exactly what we wanted to achieve.

[joquarky]

More like span, since the default is inline