Web Components and the AOM

Frontend Moscow, October 2019

Léonie Watson, TetraLogical

Browser mechanics

HTML > DOM > Accessibility Tree > User Interface

HTML: the `<button>` element

<button>Play</button>

DOM: the `<button>` element

<button>Play</button>

Acc tree: the `<button>` element

Role: button
Name: "Play"
State: focusable

UI: the `<button>` element

Custom Element: customised

class ToggleButton extends HTMLButtonElement {
    constructor() {
        super();
    }
}

customElements.define("toggle-button", ToggleButton, { extends: "button" });

connectedCallback()

connectedCallback() {
    this.setAttribute("aria-pressed", "false");
    this.addEventListener("click", togglePressed);
}

Function: the `<toggle-button>` element

function togglePressed() {
    if (this.getAttribute("aria-pressed") == "false") {
        this.setAttribute("aria-pressed", "true");
    } else {
        this.setAttribute("aria-pressed", "false");
    }
}

HTML: the `is` attribute

<button is="toggle-button">Play</button>

DOM: the `is` attribute

<button is="toggle-button" aria-pressed="false">Play</button>

Acc tree: the `is` attribute

Role: toggle-button
Name: "Play"
State: focusable

Demo: the `is="custom-button"` attribute

Custom Element: autonomous

class ToggleButton extends HTMLElement {
    constructor() {
        super();
    }
}

customElements.define("toggle-button", ToggleButton);

connectedCallback()

connectedCallback() {
    this.setAttribute("role", "button");
    this.setAttribute("tabindex", "0");
    this.setAttribute("aria-pressed", "false");

    this.addEventListener("click", togglePressed);
    this.addEventListener("keydown", function (e) {
        if (e.keyCode == 13 || e.keyCode == 32) {
            togglePressed();
        }
    });
}

HTML: the `<toggle-button>` element

<toggle-button>Play</toggle-button>

DOM: the `<toggle-button>` element

<toggle-button role="button" tabindex="0" aria-pressed="false">Play</toggle-button>

Acc tree: the `<toggle-button>` element

Role: toggle button
Name: "Play"
State: focusable

Demo: the `<toggle-button>` element

Accessibility Object Model (AOM)

Web Platform Incubator Community Group (WICG)
https://wicg.github.io/aom

Phase 1a: ARIA attribute reflection

Will enable reflection of:

ARIA attributes
Element references

Reflecting ARIA attributes

Adds two interface mixins to ARIA 1.2

AccessibleRole
AccessibleAttributes

connectedCallback()

connectedCallback() {
    this.role = "button";
    this.tabIndex = "0";
    this.ariaPressed = "false";

    ...
}

ARIA attribute reflection: implementation status

https://w3c-test.org/wai-aria/idlharness.window.html


Chrome	Edge (Chromium beta)	Firefox	Safari
82/106	82/106	1/106	86/106

Reflecting element references ⚠️

connectedCallback() {
    this.ariaDescribedByElements = [description1];
    ...
}

Phase 1b: Custom element semantics ⚠️

Will enable semantics to be provided in two ways:

Default semantics via ElementDefinitionOptions
Per instance semantics via ElementInternals

Default semantics ⚠️

class ToggleButton extends HTMLElement {
    constructor() {
        super();
    }
}
customElements.define("toggle-button", ToggleButton, { role: "button", tabIndex: "0" });

Per instance semantics ⚠️

class ToggleButton extends HTMLElement {
    var internals = null;
	constructor() {
        super();
        this.internals = customElements.createInternals(this);
        this.internals.ariaPressed = "false";
    }
}

customElements.define("toggle-button", ToggleButton, { role: "button", tabIndex: "0" });

Phase 2: User action events ⚠️

Will enable listening for Assistive Technology (AT) events

Synthesized DOM events ⚠️

To protect AT user privacy, AT events will trigger synthesized DOM events

AT click fires a synthesized click event
AT select fires a synthesized click event
AT focus will fire a synthesized focus event

New AT events ⚠️

New AT events will be introduced:

Increment/decrement
Scroll up/down
Dismiss

Phase 3: Virtual accessibility nodes ⚠️

Will allow virtual nodes to be added to the accessibility tree

Attach an accessible node to a DOM node with attachAccessibleRoot()
No mixing of DOM and virtual accessibility tree branches

Phase 4: Computed tree introspection ⚠️

Will provide access to:

Computed accessibility properties for accessibility nodes associated with DOM nodes
Computed accessibility tree including virtual accessibility nodes

HTML: the `<speak-content>` element

<speak-content>
    <span slot="control">Speak this</span>
    <div slot="speak">1 tequila... 2 tequila... 3 tequila... floor!</div>
</speak-content>

HTML template + the shadow DOM

class SpeakContent extends HTMLElement {
    constructor() {
        super();
        this.attachShadow({ mode: "open" });
        this.shadowRoot.appendChild(template.content.cloneNode(true));
    }
}
const template = document.createElement("template");
template.innerHTML = `<style> ... </style>
                      <button id="control">
                        <slot name="control"></slot>
                      </button>
                      <slot name="speak" id="speak"></slot>`;

customElements.define("speak-content", SpeakContent);

connectedCallback()

   connectedCallback() {
        var control = this.shadowRoot.querySelector("button");
        control.setAttribute("aria-pressed", "false");
        var speak = this.querySelector("div");
        var speakContent = speak.textContent;
        control.addEventListener("click", speakThis);
    }

speakThis()

    function speakThis(e) {
        if (control.getAttribute("aria-pressed") == "false") {
            control.setAttribute("aria-pressed", "true");

            var utterance = new SpeechSynthesisUtterance();
            utterance.text = speakContent;
            window.speechSynthesis.speak(utterance);

            setTimeout(resetPressedState, 500);
        }
    }

resetPressedState()

    function resetPressedState() {
        if (window.speechSynthesis.speaking === false) {
            control.setAttribute("aria-pressed", "false");
        }
        else {
            setTimeout(resetPressedState, 500);
        }
    }

CSS attribute selector

const template = document.createElement("template");
template.innerHTML = 
    `<style>
    ...
    #control[aria-pressed="true"] {
        color: #fff;
        padding: 11px 12px 9px 16px;
        text-shadow: 0 -1px 0 #444, 0 0 5px #ffd, 0 0 8px #fff;
        box-shadow: 0 1px 0 #666, 0 2px 0 #444, 0 2px 2px rgba(0, 0, 0, 0.9);
    }
    ...
    </style>`;

Demo: the `<speak-content>` element

With thanks to

Scott O'Hara @ScottOHara
Patrick Lauke @Patrick_H_Lauke
https://codepen.io/patrickhlauke/pen/byWPMX :)
Dan Hopkins @PerlBod