Custom elements make HTML the portability boundary

Reusable interface code travels farther when consumers receive an HTML element instead of framework-specific component machinery.

Custom Elements register named elements with browser-managed construction, connection, attributes, and lifecycle behavior.

The HTML Standard defines this shared platform contract across current browser engines.

Portability still depends on documented properties, attributes, methods, events, slots, semantics, and supported browser versions.

When contracts stay explicit, boundaries contain internals without hiding all integration work.

A custom element therefore reduces framework coupling, but never guarantees seamless behavior inside every rendering system.

Treat standard HTML integration as the boundary, then test each framework adapter and server-rendering path directly.