Fancy tabs web component - shadow dom v1, custom elements v1, full a11y

fancy-tabs-demo.html

<script src="https://unpkg.com/@webcomponents/custom-elements"></script>

<style>
  body {
    margin: 0;
  }
  /* Style the element from the outside */
  /*
  fancy-tabs {
    margin-bottom: 32px;
    --background-color: black;
  }*/
</style>

<fancy-tabs background>
  <button slot="title">Tab 1</button>
  <button slot="title" selected>Tab 2</button>
  <button slot="title">Tab 3</button>
  <section>content panel 1</section>
  <section>content panel 2</section>
  <section>content panel 3</section>
</fancy-tabs>

<!-- Using <a> instead of button still works! -->
<!-- <fancy-tabs background>
  <a slot="title">Title 1</a>
  <a slot="title" selected>Title 2</a>
  <a slot="title">Title 3</a>
  <section>content panel 1</section>
  <section>content panel 2</section>
  <section>content panel 3</section>
</fancy-tabs> -->

<script>
  (function () {
    'use strict';

    // Feature detect
    if (!(window.customElements && document.body.attachShadow)) {
      document.querySelector('fancy-tabs').innerHTML =
        "<b>Your browser doesn't support Shadow DOM and Custom Elements v1.</b>";
      return;
    }

    // See https://www.w3.org/TR/wai-aria-practices-1.1/#tabpanel

    customElements.define(
      'fancy-tabs',
      class extends HTMLElement {
        #shadowRoot;
        #tabsSlot;
        #selected;
        #boundOnTitleClick;
        #boundOnKeyDown;

        panels = [];
        tabs = [];

        constructor() {
          super(); // always call super() first in the ctor.

          // Create shadow DOM for the component.
          this.#shadowRoot = this.attachShadow({ mode: 'open' });
          this.#shadowRoot.innerHTML = `
            <style>
              :host {
                display: inline-block;
                width: 100%;
                font-family: 'Roboto Slab';
                contain: content;
              }
              :host([background]) {
                background: var(--background-color, #9E9E9E);
                border-radius: 10px;
                padding: 10px;
              }
              #panels {
                box-shadow: 0 2px 2px rgba(0, 0, 0, .3);
                background: white;
                border-radius: 3px;
                padding: 16px;
                height: 250px;
                overflow: auto;
              }
              #tabs {
                display: inline-flex;
                -webkit-user-select: none;
                user-select: none;
              }
              #tabs slot {
                display: inline-flex; /* Safari bug. Treats <slot> as a parent */
                gap: 4px;
              }
              /* Safari does not support #id prefixes on ::slotted
                See https://bugs.webkit.org/show_bug.cgi?id=160538 */
              #tabs ::slotted(*) {
                font: 400 16px/22px 'Roboto';
                padding: 16px 8px;
                margin: 0;
                text-align: center;
                width: 100px;
                text-overflow: ellipsis;
                white-space: nowrap;
                overflow: hidden;
                cursor: pointer;
                border-top-left-radius: 3px;
                border-top-right-radius: 3px;
                background: linear-gradient(#fafafa, #eee);
                border: none; /* if the user users a <button> */
              }
              #tabs ::slotted([aria-selected="true"]) {
                font-weight: 600;
                background: white;
                box-shadow: none;
              }
              #tabs ::slotted(:focus) {
                z-index: 1; /* make sure focus ring doesn't get buried */
              }
              #panels ::slotted([aria-hidden="true"]) {
                display: none;
              }
            </style>
            <div id="tabs">
              <slot id="tabsSlot" name="title"></slot>
            </div>
            <div id="panels">
              <slot id="panelsSlot"></slot>
            </div>
          `;
        }

        get selected() {
          return this.#selected;
        }

        set selected(idx) {
          this.#selected = idx;
          this.selectTab(idx);

          // Updated the element's selected attribute value when
          // backing property changes.
          this.setAttribute('selected', idx);
        }

        connectedCallback() {
          this.setAttribute('role', 'tablist');

          this.#tabsSlot = this.#shadowRoot.querySelector('#tabsSlot');
          const panelsSlot = this.#shadowRoot.querySelector('#panelsSlot');

          this.tabs = this.#tabsSlot.assignedNodes({ flatten: true });
          this.panels = panelsSlot
            .assignedNodes({ flatten: true })
            .filter((el) => el.nodeType === Node.ELEMENT_NODE);

          // Add aria role="tabpanel" to each content panel.
          for (const panel of this.panels) {
            panel.setAttribute('role', 'tabpanel');
            panel.setAttribute('tabindex', 0);
          }

          // Referernces to we can remove listeners later.
          this.#boundOnTitleClick = this.#onTitleClick.bind(this);
          this.#boundOnKeyDown = this.#onKeyDown.bind(this);

          this.#tabsSlot.addEventListener('click', this.#boundOnTitleClick);
          this.#tabsSlot.addEventListener('keydown', this.#boundOnKeyDown);

          this.selected = this.#findFirstSelectedTab() || 0;
        }

        disconnectedCallback() {
          this.#tabsSlot.removeEventListener('click', this.#boundOnTitleClick);
          this.#tabsSlot.removeEventListener('keydown', this.#boundOnKeyDown);
        }

        #onTitleClick(e) {
          if (e.target.slot === 'title') {
            this.selected = this.tabs.indexOf(e.target);
            e.target.focus();
          }
        }

        #onKeyDown(e) {
          switch (e.code) {
            case 'ArrowUp':
            case 'ArrowLeft':
              e.preventDefault();
              var idx = this.selected - 1;
              idx = idx < 0 ? this.tabs.length - 1 : idx;
              this.tabs[idx].click();
              break;
            case 'ArrowDown':
            case 'ArrowRight':
              e.preventDefault();
              var idx = this.selected + 1;
              this.tabs[idx % this.tabs.length].click();
              break;
            default:
              break;
          }
        }

        #findFirstSelectedTab() {
          let selectedIdx;
          for (let [i, tab] of this.tabs.entries()) {
            tab.setAttribute('role', 'tab');

            // Allow users to declaratively select a tab
            // Highlight last tab which has the selected attribute.
            if (tab.hasAttribute('selected')) {
              selectedIdx = i;
            }
          }
          return selectedIdx;
        }

        selectTab(idx = null) {
          for (let [i, tab] of this.tabs.entries()) {
            const select = i === idx;
            tab.setAttribute('tabindex', select ? 0 : -1);
            tab.setAttribute('aria-selected', select);
            this.panels[i].setAttribute('aria-hidden', !select);
          }
        }
      }
    );
  })();
</script>

<div id="app"></div>

<!-- Using <a> instead of h2 still works! -->

not h2 - button