Accessibility Scene Tree for Screen Readers

What is Screen Reader?

Screen reader is an assistive technology for blind or low vision people to interact with digital content. For users with blindness or low vision, they can't rely 100% on visual interface. Depending on the degree of visual disabilities, some of them don't even use a monitor display or mouse when they use computers. This user would use screen reading software to translate what is displayed on screen into speech or braille output. They use keyboard to tell the screen reader what to read and interact with the computer. Here's an example video of how a user uses screen reader: Screen Reader Demo - YouTube.

There are different screen reader applications that a user can choose, including OS built-in screen readers like Windows's "Narrator", MacOS and iOS's "VoiceOver", Android's "TalkBack", and third party screen readers like JAWS and NVDA.

Nowadays 2D pages on web generally have good accessibility support to screen reader users, because it can understand different HTML elements. But 3D applications like WebGL applications do not. This is because objects in webGL applications are rendered in a <canvas> element. When screen reader read the page, it will only read: "Image", but can't interpret any objects inside the scene. If we do not deal with it correctly, users who are blind or low vision will have difficulty using the application.

The Accessibility Package provides a way to create html twin elements for objects in the scene that should be accessible. Here's an example of a simple scene of three boxes using the Accessibility Package. If you turn on a screen reader to read the page, it will say "A big box in the middle of the scene. A small box on the left of the big box. A small box on the right of the big box".

Boxes with accessible description

How to use Accessibility Package to Support Screen Reader and Keyboard Navigation


Blind or low vision users listen to the content and use keyboard to interact, so we need to add description to Babylon.js content that will be read by screen reader. You can add description to your Nodes or Controls (that you think should be accessible) by using IAccessibilityTag.

let egg = BABYLON.MeshBuilder.CreateSphere("Egg", { diameterX: 0.62, diameterY: 0.8, diameterZ: 0.6 }, scene);
egg.accessibilityTag = {
description: "An easter egg",

Not all content in the scene should be accessible. For example, the decorative trees or background image on a UI panel. Only add IAccessibilityTag to the contents that's important to the user experience.

By default, all Controls (GUI) are considered "important" for accessibility, and will have their html twins with its own information (like the text to show on a button), even if you haven't assigned the IAccessibility tag. But if you define the IAccessibilityTag, it can override default metadata (like assigning the IAccessibilityTag.description will override the text to show on a button). By default, Node type objects are not considered "important", unless you assign a IAccessibilityTag to it.


The accessibility package basically generates HTML elements for your scene, based on the metadata (IAccessibilityTag) you added to your scene content. The screen reader then can read the HTML elements. To generate it for your scene:


This will generate a <div id="accessibility-host"> HTML element right after the Babylon.js scene's canvas element. Inside this div element, the renderer generates HTML twin elementss for each of your accessible contents in the scene. These HTML twin elements are internally connected with Babylon.js objects, so the screen reader user can also interact with the Babylon.js objects through their HTML twin.


Some of your contents in the scene might be interactable (e.g. clickable). For Node type objects, if you use Babylon.js's ActionManager to define the interaction, the Accessibility package can automatically detect it and apply on the generated HTML twin elements, so that the user can use keyboard to trigger events like click or right click on the HTML twin elements, thus trigger the correspond action on the Babylon.js objects. Only ACTION_OnPickTrigger, ACTION_OnLeftPickTrigger, ACTION_OnRightPickTrigger are supported. For Control type objects, if you defined observer for onPointerClickObservable, the Accessibility package can also automatically detect it and apply on the generated HTML twin element.

If you want to customize the interaction, use the eventHandler field of IAccessibilityTag:

let egg = BABYLON.MeshBuilder.CreateSphere("Egg", { diameterX: 0.62, diameterY: 0.8, diameterZ: 0.6 }, scene);
egg.accessibilityTag = {
description: "An easter egg",
eventHandler: {
onclick: yourFunction,
Custom event handling

When is HTML twins Updated

Your scene might be not static, and you may want to update html twins when your scene is changed. The html twins will automatically update when:

  • A Node (Mesh or TransformNode) is added/removed in a scene;
  • A Node (Mesh or TransformNode)'s enabled status is changed;
  • A Control is added/removed from a Container;
  • A Control's isVisible status is changed;
  • A Node or Control's IAccessibilityTag is assigned or re-assigned;
Update Accessibility Tree

Customize HTML Twin with ARIA Attributes

If you are a pro in Web Accessibility, and know what you are doing, you can use IAccessibilityTag.role and IAccessibilityTag.aria to assign different Role and ARIA attributes to the HTML Twin of this object to do whatever you want.

yourObject.accessibilityTag = {
description: "An demo customized progressbar",
role: "progressbar",
aria: {
"aria-valuemin": "0",
"aria-valuemax": "100"
"aria-valuenow": "0"

Note that using ARIA attributes incorrectly can introduce errors in your webpage. While ARIA is designed to make web pages more accessible, if used incorrectly, it can do more harm than good. If you choose to use ARIA, you are responsible for mimicking the equivalent browser behavior in script.