UA Widgets


User Agent Widgets (UA Widgets) are intended to be a replacement of our usage of XBL bindings in web content. These widgets run JavaScript inside extended principal per-origin sandboxes. They insert their own DOM inside of a special, closed Shadow Root inaccessible to the page, called a UA Widget Shadow Root.

UA Widget lifecycle

UA Widgets are generally constructed when the element is appended to the document and destroyed when the element is removed from the tree. Yet, in order to be fast, specialization was made to each of the widgets.

When the element is appended to the tree, a chrome-only UAWidgetSetupOrChange event is dispatched and is caught by a frame script, namely UAWidgetsChild.

UAWidgetsChild then grabs the sandbox for that origin (lazily creating it as needed), loads the script as needed, and initializes an instance by calling the JS constructor with a reference to the UA Widget Shadow Root created by the DOM. We will discuss the sandbox in the latter section.

The onsetup method is called right after the instance is constructed. The call to constructor must not throw, or UAWidgetsChild will be confused since an instance of the widget will not be returned, but the widget is already half-initalized. If the onsetup method call throws, UAWidgetsChild will still be able to hold the reference of the widget and call the teardown method later on.

When the element is removed from the tree, UAWidgetTeardown is dispatched so UAWidgetsChild can destroy the widget, if it exists. If so, the UAWidgetsChild calls the teardown() method on the widget, causing the widget to destruct itself.

Counter-intuitively, elements are not considered “removed from the tree” when the document is unloaded. This is considered safe as anything the widget touches should be reset or cleaned up when the document unloads. Please do not violate the assumption by having any browser state toggled by the teardown.

When a UA Widget initializes, it should create its own DOM inside the passed UA Widget Shadow Root, including the <link> element necessary to load the stylesheet, add event listeners, etc. When destroyed (i.e. the teardown method is called), it should do the opposite.

Specialization: for video controls, we do not want to do the work if the control is not needed (i.e. when the <video> or <audio> element has no “controls” attribute set), so we forgo dispatching the event from HTMLMediaElement in the BindToTree method. Instead, another UAWidgetSetupOrChange event will cause the sandbox and the widget instance to construct when the attribute is set to true. The same event is also responsible for triggering the onchange() method on UA Widgets if the widget is already initialized.

Likewise, the datetime box widget is only loaded when the type attribute of an <input> is either date or time.

The specialization does not apply to the lifecycle of the UA Widget Shadow Root. It is always constructed in order to suppress children of the DOM element from the web content from receiving a layout frame.

UA Widget Shadow Root

The UA Widget Shadow Root is a closed shadow root, with the UA Widget flag turned on. As a closed shadow root, it may not be accessed by other scripts. It is attached on host element which the spec disallow a shadow root to be attached.

The UA Widget flag enables the security feature covered in the next section.

Side note: XML pretty print hides its transformed content inside a UA Widget Shadow DOM as well, even though there isn’t any JavaScript to run. This is set in order to leverage the same security feature and behaviors there.

The JavaScript sandbox

The sandboxes created for UA Widgets are per-origin and set to the expanded principal. This allows the script to access other DOM APIs unavailable to the web content, while keeping its principal tied to the document origin. They are created as needed, backing the lifecycle of the UA Widgets as previously mentioned. These sandbox globals are not associated with any window object because they are shared across all same-origin documents. It is the job of the UA Widget script to hold and manage the references of the window and document objects the widget is being initiated on, by accessing them from the UA Widget Shadow Root instance passed.

While the closed shadow root technically prevents content from accessing the contents, we want a stronger guarantee to protect against accidental leakage of references to the UA Widget shadow tree into content script. Access to the UA Widget DOM is restricted by having their reflectors set in the UA Widgets scope, as opposed to the normal scope. To accomplish this, we avoid having any script (UA Widget script included) getting a hold of the reference of any created DOM element before appending to the Shadow DOM. Once the element is in the Shadow DOM, the binding mechanism will put the reflector in the desired scope as it is being accessed.

To avoid creating reflectors before DOM insertion, the available DOM interfaces are limited. For example, instead of createElement() and appendChild(), the script would have to call createElementAndAppendChildAt() available on the UA Widget Shadow Root instance, to avoid receiving a reference to the DOM element and thus triggering the creation of its reflector in the wrong scope, before the element is properly associated with the UA Widget shadow tree. To find out the differences, search for Func="IsChromeOrUAWidget" and Func="IsNotUAWidget" in in-tree WebIDL files.

Other things to watch out for

As part of the implementation of the Web Platform, it is important to make sure the web-observable characteristics of the widget correctly reflect what the script on the web expects.

  • Do not dispatch non-spec compliant events on the UA Widget Shadow Root host element, as event listeners in web content scripts can access them.

  • The layout and the dimensions of the widget should be ready by the time the constructor and onsetup returns, since they can be detectable as soon as the content script gets the reference of the host element (i.e. when appendChild() returns). In order to make this easier we load <link> elements load chrome stylesheets synchronously when inside a UA Widget Shadow DOM.

  • There shouldn’t be any white-spaces nodes in the Shadow DOM, because UA Widget could be placed inside white-space: pre. See bug 1502205.

  • CSP will block inline styles in the Shadow DOM. <link> is the only safe way to load styles.