Elegant
element

Element

Elements have an associated namespace, namespace prefix, local name, custom element state, custom element definition, is value. When an element is created, all of these values are initialized.

An element’s custom element state is one of "undefined", "failed", "uncustomized", "precustomized", or "custom". An element whose custom element state is "uncustomized" or "custom" is said to be defined. An element whose custom element state is “custom” is said to be custom.

The following code illustrates elements in each of these four states:

<!DOCTYPE html>
<script>
  window.customElements.define("sw-rey", class extends HTMLElement {})
  window.customElements.define("sw-finn", class extends HTMLElement {}, { extends: "p" })
  window.customElements.define("sw-kylo", class extends HTMLElement {
    constructor() {
      // super() intentionally omitted for this example
    }
  })
</script>

<!-- "undefined" (not defined, not custom) -->
<sw-han></sw-han>
<p is="sw-luke"></p>
<p is="asdf"></p>

<!-- "failed" (not defined, not custom) -->
<sw-kylo></sw-kylo>

<!-- "uncustomized" (defined, not custom) -->
<p></p>
<asdf></asdf>

<!-- "custom" (defined, custom) -->
<sw-rey></sw-rey>
<p is="sw-finn"></p>

References