:defined

In this demo, we define a basic custom element named <custom-element> and use the :not(:defined) and :defined selectors to style the element before and after it is defined. This is useful if you have a complex custom element that takes a while to load into the page — you might want to hide instances of the element until the definition is complete so that you don't end up with flashes of ugly unstyled elements on the page.

HTML

The following HTML code uses the custom element but the element hasn't been defined yet. We also include a <button> that will define the custom element when clicked, allowing you to see its state before and after definition.

<custom-element>
  <p>
    Loaded content: Lorem ipsum tel sed tellus eiusmod tellus. Aenean. Semper
    dolor sit nisi. Elit porttitor nisi sit vivamus.
  </p>
</custom-element>

<button id="btn">define the <code>&lt;custom-element&gt;</code></button>

CSS

custom-element {
  display: block;
  border: 5px dashed grey;
  border-radius: 1rem;
  height: 100px;
  width: 400px;
  padding: 1rem;
  position: relative;
  user-select: none;
}

code {
  background: #ccc;
}

#btn {
  margin-top: 1rem;
  cursor: pointer;
}

In the following CSS, we use the custom-element:not(:defined) selector to select the element and color it grey while it is not defined and the custom-element:defined selector to select the element and color it black after it is defined.

custom-element:not(:defined) {
  border-color: grey;
  color: grey;
}

custom-element:defined {
  background-color: wheat;
  border-color: black;
  color: black;
}

/* show loading message */
custom-element:not(:defined)::before {
  content: "Loading...";
  position: absolute;
  inset: 0 0 0 0;
  align-content: center;
  text-align: center;
  font-size: 2rem;
  background-color: white;
  border-radius: 1rem;
}

/* remove the loading message */
custom-element:defined::before {
  content: "";
}

We have also used the ::before pseudo-element to show a "Loading..." overlay message until the element is defined. After definition, it is removed by setting the content to an empty string.

The following JavaScript has been used to define the custom element. To allow you to see the state of the custom element before and after definition we run the define() method when the button is clicked.

const btn = document.querySelector("#btn");

btn.addEventListener("click", () => {
  customElements.define("custom-element", class extends HTMLElement {});
  btn.remove();
});

Result

• Web components