Web Components: Introducing Custom Elements

One of the new features in Safari Technology Preview 18 is Custom Elements v1 (Chrome/Opera already support for it):

To define a custom element, simply invoke customElements.define with a new local name of the element and a subclass of HTMLElement.

Example Code:

class CustomProgressBar extends HTMLElement {
  constructor() {
      super();
      const shadowRoot = this.attachShadow({mode: 'closed'});
      shadowRoot.innerHTML = `
          <style>
              :host { display: inline-block; width: 5rem; height: 1rem; }
              .progress { display: inline-block; position: relative; border: solid 1px #000; padding: 1px; width: 100%; height: 100%; }
              .progress > .bar { background: #9cf; height: 100%; }
              .progress > .label { position: absolute; top: 0; left: 0; width: 100%;
                  text-align: center; font-size: 0.8rem; line-height: 1.1rem; }
          </style>
          <div class="progress" role="progressbar" aria-valuenow="0" aria-valuemin="0" aria-valuemax="100">
              <div class="bar" style="width: 0px;"></div>
              <div class="label">0%</div>
          </div>
      `;
      this._progressElement = shadowRoot.querySelector('.progress');
      this._label = shadowRoot.querySelector('.label');
      this._bar = shadowRoot.querySelector('.bar');
  }

  get progress() { return this._progressElement.getAttribute('aria-valuenow'); }
  set progress(newPercentage) {
      this._progressElement.setAttribute('aria-valuenow', newPercentage);
      this._label.textContent = newPercentage + '%';
      this._bar.style.width = newPercentage + '%';
  }
};
customElements.define('custom-progress-bar', CustomProgressBar);

Once defined you can then use it as follows in your HTML:

<custom-progress-bar"></custom-progress-bar>

It is also possible to create instances using JavaScript, and to define the allowed attributes for custom elements (and listen for changes on them).

Introducing Custom Elements →
Custom Element Demo →