interface XRControllerBinding {
  readonly attribue DOMString binding;
  readonly attribue DOMString[] actions;
  readonly attribue DOMString valueType; // "boolean, double, Float32Array"
}

partial interface XRInputSource {
  readonly attribue DOMString id;
  readonly attribue sequence<XRControllerBinding> bindings;
  readonly object getBindings();

  // If it fails to bind actions, for example there's no this binding, it returns false.
  boolean registerActionBinding(DOMString action, DOMString binding);

  boolean getAction(DOMString action);
  double getAction(DOMString action);
  short getAction(DOMString action);
  Float32Array getAction(DOMString action);

  void setAction(DOMString action, double value);
  void setAction(DOMString action, boolean value);
  void setAction(DOMString action, Float32Array value);
};

--------------
In Javascript:

function init() {
  let input = xrSession.getInputSources()[0];
  let bindings = input.bindings;

  // The binding format would be
  // "bodyPart: {hand, foot, etc}"/
  // "side: {left or right}"/
  // "from: {input or output}"/
  // "item: {btn, axis, joystick, trackpad}"/
  // "eventType: {click, touch, value}"
  input.registerActionBinding("menu_select", "hand/left/input/btn_A/click");
  input.registerActionBinding("trigger_press", "hand/left/input/trigger/value");
  input.registerActionBinding("pose_value", "hand/left/input/pose/value");
  input.registerActionBinding("bone_count", "hand/left/input/skeleton/count");
  input.registerActionBinding("bone_matrix", "hand/left/input/skeleton/transform");
  input.registerActionBinding("haptics_value", "hand/left/output/haptics/value");

  console.log(input.getBindings());
}
---
{
  {
    "binding", "hand/left/input/btn_A/click",
    "actions" : ["menu_select"],
    "valueType", "boolean"
  },
  {
    "binding", "hand/left/input/btn_A/touched",
    "valueType", "boolean"
  },
  {
    "binding", "hand/left/input/trigger/value",
    "actions" : ["trigger_press"],
    "valueType", "double"
  },
  {
    "binding", "hand/left/input/pose/value",
    "actions" : ["pose_value"],
    "valueType", "Float32Array"
  },
  {
    "binding", "hand/left/input/bone/count",
    "actions" : ["bone_count"],
    "valueType", "short"
  },
  {
    "binding", "hand/left/input/bone/transform",
    "actions" : ["bone_matrix"],
    "valueType", "Float32Array"
  },
  {
    "binding", "hand/left/output/haptics/value",
    "actions" : ["haptics_value"],
    "valueType", "Float32Array"
  }
}
---

function gameloop() {
  if (input.getAction("menu_select") == true) {
    // ...
  }

  let trigger_value = input.getAction("trigger_value");
  let pose = input.getAction("pose_value");
  let bone_count = input.getAction("bone_count");
  let bone_matrix = input.getAction("bone_matrix");

  input.setAction("haptics_value", new Float32Array([0.1, 3000]));
}