Os nossos voluntários ainda não traduziram este artigo para Português (Europeu). Junte-se a nós e ajude-nos a fazer o trabalho!
Pode também ler o artigo em English (US).

The accessibility inspector provides a means to access important information exposed to assistive technologies on the current page via the accessibility tree, allowing you to check what's missing or otherwise needs attention. This article takes you through the main features of the accessibility inspector and how to use it.

A (very) brief guide to accessibility

Accessibility is the practice of making your websites usable by as many people as possible. This means trying your best to not lock anyone out of accessing information because of any disability they may have, or any other personal circumstances such as the device they are using, the speed of their network connection, or their geographic location or locale.

Here we are mainly talking about about exposing information to people with visual disabilities — this is done via the accessibility APIs available inside web browsers, which expose information on what roles the different elements on your page play (e.g., are they just text, or are they buttons, links, form elements, etc.?).

Semantic DOM elements have roles assigned to them by default that hint at what their purpose is. Sometimes, however, you need to use some non-semantic markup (e.g., <div>s) to build a complex custom control, and the control won't have a default role that reflects its purpose. In such a situation, you can use WAI-ARIA role attributes to provide your own roles.

Roles and other information exposed by browser accessibility APIs is presented in a hierarchical structure called the accessibility tree. This is a bit like the DOM tree, except that it contains a more limited set of elements and slightly different information about them.

Assistive technologies like screenreaders use this information to find out what's on a web page, tell their users what's there, and enable them to interact with the page. The accessibility inspector also uses this information to provide valuable accessibility debugging capabilities in the DevTools.

Accessing the accessibility inspector

The accessibility inspector (available since Firefox 61) is not shown by default in the DevTools. To turn it on, you need to go to DevTools Settings (press F1, or go to the "three dots" menu and choose Settings) and check the Accessibility checkbox under the Default Developer Tools heading. This will cause the Accessibility tab to appear in the main DevTools display, which can be clicked on to display the accessibility panel:

Accessibility tab in firefox devtools, turned off, with a button labeled Turn On Accessibility Features

Initially the DevTools accessibility features are turned off (unless you've already got them turned on in another browser tab, or got the Firefox accessibility engine started already, e.g., you might be a screenreader user or tester). This is because the accessibility engine runs in the background when the accessibility features are turned on. While it’s running, it slows performance and takes up memory; therefore it interferes with the metrics from other panels such as Memory and Performance as well as overall browser performance. For this reason you should keep it turned off when you aren't specifically using it.

You can turn the features on using the Turn On Accessibility Features button.

Once the panel content loads, you can then turn it off again using the Turn Off Accessibility Features button available in the top-left corner, unless you have the accessibility engine running previously to operate a screenreader, in which case this button will be disabed.

Note: If are using the accessibility features in multiple tabs, turning them off in one tab turns them off in all tabs.

Features of the Accessibility panel

The enabled accessibility panel looks like so:

Accessibility tab in firefox devtools, turned on, showing two information panels plus a button labeled Turn Off Accessibility Features

On the left-hand side, there is a tree diagram representing all the items in the accessibility tree for the current page. Items with nested children have arrows that can be clicked to reveal the children, so you can move deeper into the hierarchy. Each item has two properties listed:

  • Role — the role this item has on the page (e.g., pushbutton, or footer). This can be either a default role provided by the browser, or a role given to it via a WAI-ARIA role attribute.
  • Name — the name this item has on the page. Where this comes from depends on the element; for example, the name of most text elements is simply their textContent, whereas form elements' names are the contents of their associated <label>.

On the right-hand side, you can see further information about the currently selected item. The listed properties are as follows:

  • name — the item's name, as described above.
  • role — the item's role, as described above.
  • actions — a list of the actions that can be performed on the item, for example a pushbutton would have "Press" listed, while a link would have "Jump" listed.
  • value — the value of the item. This can mean different things depending on the type of item; for example, a form input (role: entry) would have a value of whatever is entered in the input, whereas a link's value would be the URL in the corresponding <a> element's href.
  • DOMNode — the type of DOM node that the item in the accessibility tree represents. You can click on the "target" icon that comes after it to select the node in the Page Inspector. Hovering over the "target" icon highlights the DOM node in the page content.
    DOMNode property in accessibility inspector with target icon highlighted
  • description — any further description provided on the element, usually by the content of a title attribute.
  • help — this is not implemented in Gecko, so it always returns an empty string. This will be removed from the inspector in Firefox 62 (bug 1467643).
  • keyboardShortcut — any keyboard shortcut that is available to activate the element, as specified in an accessKey attribute. Note that this works correctly as of Firefox 62 (bug 1467381).
  • childCount — the number of child items the current item has in the accessibility tree hierarchy.
  • indexInParent — an index value indicating what number child the item is, inside its parent. If the item is the first item inside its parent, it has a value of 0. If it is the second, it has a value of 1. And so on.
  • states — a list of the different accessibility-relevant states that can apply to the current item. For example, one of the links in one demo has states of focusable, linked, selectable text, opaque, enabled, and sensitive. For a full list of internal states, see Gecko states.
  • attributes — a list of all the accessibility-relevant attributes that are applied to the item. This can include style-related attributes such as margin-left and text-indent, and other useful states for accessibility information, such as draggable and level (e.g., what heading level is it, in the case of headings). For a full list of possible attributes, see Gecko object attributes.

Note: The exposed information is the same across all platforms — the inspector exposes Gecko's accessibility tree, rather than information from the platform accessibility layer.

Keyboard controls

The Accessibility tab is fully keyboard-accessible:

  • You can tab between the Turn Off Accessibility Features button and left and right panels.
  • When one of the panels is focused, you can move the focus up and down items using the up and down arrow keys, and use the left and right arrow keys to expand and collapse expandable rows (e.g., different hierarchy levels of the accessibility tree).

When the accessibility features are turned on, there are a number of useful additional features available in the DevTools, which are detailed below:

Context menu options

An extra context menu option is added, both for the general context menu on the web page when right/Ctrl + clicking a UI feature, and the HTML pane of the page inspector when right/Ctrl + clicking a DOM element:

context menu in the browser viewport, with a highlighted option: Inspect Accessibility Properties

context menu in the DOM inspector, with a highlighted option: Show Accessibility Properties

When you choose the Inspect Accessibility Properties/Show Accessibility Properties context menu options, the Accessibility tab is immediately opened to show the corresponding accessibility tree item and its properties.

Note: Some DOM elements do not have accessibility properties — in such a case, the Inspect Accessibility Properties/Show Accessibility Properties context menu item is grayed out.

Highlighting of UI items

In the Accessibility tab, when the mouse hovers over accessibility items, you can see a semi-transparent highlight appear over the UI items they relate to, if appropriate. This is useful for determining how the items in the accessibility tree relate to the UI items on the actual page.

Accessibility picker

In a similar way to the Page Inspector HTML pane picker, when the Accessibility tab's picker button is pressed you can then hover and select UI items in the current page, and the corresponding accessible object is highlighted in the accessibility tree.

The accessibility tab picker differs in look slightly from the Page Inspector HTML pane picker, as shown below:

highlighted dom inspector picker button, with a tooltip saying Pick an element from the page

highlighted accessibility inspector button, with a tooltip saying Pick accessible object from the page

When you "perform a pick", you see the accessibility object highlighted in the accessibility tree, and the picker is then deactivated. Note, however, that if you hold the Shift key down when "performing a pick", you can "preview" the accessibility object in the tree (and its properties in the right-hand pane), but then continue picking as many times as you like (the picker does not get cancelled) until you release the Shift key.

When the picker is activated, you can also deactivate it by pressing the picker button a second time, or pressing the Esc key.

Typical use cases

The accessibility inspector is very useful for spotting accessibility problems at a glance. For a start, you can investigate items that don't have a proper text equivalent — images without alt text and form elements without proper labels have a name property of null, for example.

A form input highlighted in the UI, with information about it shown in the accessibility inspector to reveal that it has no label — it has a name property of null

It is also very handy for verifying semantics — you can use the Inspect Accessibility Properties context menu option to quickly see whether an item has the correct role set on it (e.g., whether a button is really a button, or whether a link is really a link).

A UI element that looks like a button, with information about it shown in the accessibility inspector to reveal that it isn't a button, it is a section element. It has a name property of null

See also

Etiquetas do documento e contribuidores

Contribuidores para esta página: wbamberg, chrisdavidmills, jswisher, yzen
Última atualização por: wbamberg,