message: docs(elements): add MDN links for mentioned built-in APIs in Elements guide
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diff: @@ -64,8 +64,7 @@ create and destroy instances, and to detect and respond to changes.
  The conversion process implements the `NgElementConstructor` interface, and creates a
  constructor class that is configured to produce a self-bootstrapping instance of your component.
  -Use a JavaScript function, `customElements.define()`, to register the configured constructor
  -and its associated custom-element tag with the browser's `CustomElementRegistry`.
  +Use the built-in [`customElements.define()`](https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry/define) function to register the configured constructor and its associated custom-element tag with the browser's [`CustomElementRegistry`](https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry).
  When the browser encounters the tag for the registered element, it uses the constructor to create a custom-element instance.
  <div class="lightbox">
  