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Yes, the article specifically states this.

The article says "without relying on a JS framework.", which is different than not requiring JS. It does not work when Javascript is disabled.

they wont display what the javascript code tells them to display, but rather fallback to a <div /> mode.

> Will the browser be able to display them without requiring JS?

Without JS (which registers the element), custom element tags will be like any other unknown tag. It's like a span unless the CSS says otherwise.

The behavior will be missing, however.