The region identified by the ID ID in a bitmap bitmap is the value returned by the following algorithm (which can return a hit region or nothing):

1. If ID is null, return nothing and abort these steps.

2. Let list be the hit region list associated with bitmap.

3. If there is a hit region in list whose ID is a case-sensitive match for ID, then return that hit region and abort these steps.

4. Otherwise, return nothing.

The region representing the control control for a bitmap bitmap is the value returned by the following algorithm (which can return a hit region or nothing):

1. Let list be the hit region list associated with bitmap.

2. If there is a hit region in list whose control is control, then return that hit region and abort these steps.

3. Otherwise, return nothing.

The control represented by a region region for a canvas element ancestor is the value returned by the following algorithm (which can return an element or nothing):

1. If region has no control, return nothing and abort these steps.

2. Let control be region's control.

3. If control is not a descendant of ancestor, then return nothing and abort these steps.

4. Otherwise, return control.

The region for a pixel pixel on a bitmap bitmap is the value returned by the following algorithm (which can return a hit region or nothing):

1. Let list be the hit region list associated with bitmap.

2. If there is a hit region in list whose set of pixels contains pixel, then return that hit region and abort these steps.

3. Otherwise, return nothing.

To clear regions that cover the pixels pixels on a bitmap bitmap, the user agent must run the following steps:

1. Let list be the hit region list associated with bitmap.

2. Remove all pixels in pixels from the set of pixels of each hit region in list.

3. Garbage-collect the regions of bitmap.

To garbage-collect the regions of a bitmap bitmap, the user agent must run the following steps:

1. Let list be the hit region list associated with bitmap.

2. Loop: Let victim be the first hit region in list to have an empty set of pixels . If there is no such hit region, abort these steps.

3. Remove victim from list.

4. Jump back to the step labeled loop.

Adding a new region and calling clearRect() are the two ways this clearing algorithm can be invoked. The hit region list itself is also reset when the rendering context is reset, e.g. when a CanvasRenderingContext2D object is bound to or unbound from a canvas, or when the dimensions of the bitmap are changed.

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When the addHitRegion() method is invoked, the user agent must run the following steps:

1. Let arguments be the dictionary object provided as the method's argument.

2. Let path be the CanvasRenderingContext2D object's current default path.

3. Let specified pixels be the pixels contained in source path, using the fill rule indicated by the fillRule member.

4. If the arguments object's id member is the empty string, let it be null instead.

5. If the arguments object's id member is not null, then let previous region for this ID be the region identified by the ID given by the id member's value in this scratch bitmap, if any. If the id member is null or no such region currently exists, let previous region for this ID be null.

6. If any of the following conditions are met, throw a NotSupportedError exception and abort these steps.

   • The specified pixels has no pixels.

7. Let region be a newly created hit region, with its information configured as follows:

   Hit region's set of pixels

   The specified pixels

   Hit region's bounding circumference

   A user-agent-defined shape that wraps the pixels contained in source path. (In the simplest case, this can just be the bounding rectangle; this specification allows it to be any shape in order to allow other interfaces.)

   Hit region's ID

   If the arguments object's id member is not null: the value of the id member. Otherwise, region has no id.

   Hit region's control

   If the arguments object's control member is not null: the value of the control member. Otherwise, region has no control.

8. If the arguments object's control member is not null, then let previous region for the control be the region representing the control given by the control member's value for this scratch bitmap, if any. If the control member is null or no such region currently exists, let previous region for the control be null.

9. If there is a previous region with this control, remove it from the scratch bitmap's hit region list .

10. If there is a previous region with this ID, remove it from the scratch bitmap's hit region list .

11. Clear regions that cover the pixels in region's set of pixels on this scratch bitmap.

12. Add region to the scratch bitmap's element's hit region list.

When the removeHitRegion() method is invoked, the user agent must run the following steps:

1. Let region be the region identified by the ID given by the method's argument in the rendering context's scratch bitmap. If no such region currently exists, abort these steps.

   If the method's argument is the empty string, then no region will match.

2. Remove region from the rendering context's scratch bitmap's hit region list .

3. Garbage-collect the regions of the rendering context's scratch bitmap.

partial interface MouseEvent {
  readonly attribute DOMString? region;
};

partial dictionary MouseEventInit {
  DOMString? region;
};

The region attribute on MouseEvent objects must return the value it was initialized to. When the object is created, this attribute must be initialized to null. It represents the hit region's ID if the mouse was over a hit region when the event was fired.

When a MouseEvent is to be fired at a canvas element by the user agent in response to a pointing device action, if the canvas element has a hit region list, the user agent must instead follow these steps. If these steps say to act as normal, that means that the event must be fired as it would have had these requirements not been applied.

1. If the pointing device is not indicating a pixel on the canvas, act as normal and abort these steps.

2. Let pixel be the pixel indicated by the pointing device.

3. Let region be the hit region that is the region for the pixel pixel on this canvas element's bitmap, if any.

4. If there is no region, then act as normal and abort these steps.

5. Let id be the region's ID, if any.

6. If there is an id, then initialize the event object's region attribute to id.

7. Let control be the control represented by region for this canvas element, if any.

8. If there is a control, then target the event object at control instead of the canvas element.

9. Continue dispatching the event, but with the updated event object and target as given in the above steps.

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When a user's pointing device cursor is positioned over a canvas element, user agents should render the pointing device cursor according to the cursor specification described by the cursor for the hit region that is the region for the pixel that the pointing device designates on the canvas element's bitmap.

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User agents are encouraged to make use of the information present in a canvas element's hit region list to improve the accessibility of canvas elements.

Each hit region should be handled in a fashion equivalent to a node in a virtual DOM tree rooted at the canvas element. The hierarchy of this virtual DOM tree must match the hierarchy of the hit regions, as described by the parent of each region. Regions without a parent must be treated as children of the canvas element for the purpose of this virtual DOM tree. For each node in such a DOM tree, the hit region's bounding circumference gives the region of the screen to use when representing the node (if appropriate).

The semantics of a hit region for the purposes of this virtual DOM tree are those of the the control represented by the region, if it has one .

For the purposes of accessibility tools, when an element C is a descendant of a canvas element and there is a region representing the control C for that canvas element's bitmap, then the element's position relative to the document should be presented as if it was that region in the canvas element's virtual DOM tree.

The semantics of a hit region for the purposes of this virtual DOM tree are those of the the control represented by the region, if it has one, or else of a non-interactive element whose ARIA role, if any, is that given by the hit region's ARIA role, and whose textual representation, if any, is given by the hit region's label.

Thus, for instance, a user agent on a touch-screen device could provide haptic feedback when the user croses over a hit region's bounding circumference, and then read the hit region's label to the user. Similarly, a desktop user agent with a virtual accessibility focus separate from the keyboard input focus could allow the user to navigate through the hit regions, using the virtual DOM tree described above to enable hierarchical navigation. When an interactive control inside the canvas element is focused, if the control has a corresponding region, then that hit region's bounding circumference could be used to determine what area of the display to magnify.