Инспектор доступности предоставляет средства для доступа к важной информации, предоставляемой вспомогательным технологиям на текущей странице, через дерево доступности, позволяя вам проверить, что отсутствует или иным образом требует внимания. В этой статье вы узнаете основные функции инспектора доступности и способы его использования.
Доступность - это средство, позволяющее использовать ваши сайты как можно большему числу людей. Это означает, что вы стараетесь изо всех сил не ограничивать кого-либо в доступе к информации из-за какой-либо имеющейся инвалидности, или любых других личных особенностей, таких как используемое устройство, скорости их сетевого подключения или их географического местоположения или языка.
Здесь мы в основном говорим о том, чтобы донести информацию людям с нарушениями зрения - это делается с помощью API доступности, доступных внутри веб-браузеров, которые раскрывают информацию о том, какие роли играют разные элементы на вашей странице (например, они являются текстом или кнопками, ссылками, элементами форм и т. д.?).
Семантические элементы DOM имеют роли, назначенные им по умолчанию, которые имеют своё предназначение. Иногда приходится использовать некоторую не семантическую разметку (например, {{htmlelement ("div")}}) для создания сложного настраиваемого элемента управления, а элемент управления не будет иметь роль по умолчанию, которая отражает его назначение. В такой ситуации вы можете использовать атрибуты роли WAI-ARIA для обозначения своих собственных ролей.
Роли и другая информация, предоставляемая API-интерфейсами браузера, представлена в иерархической структуре, называемой деревом доступности. Это немного похоже на дерево DOM, за исключением того, что оно содержит более ограниченный набор элементов и немного другую информацию о них.
Вспомогательные технологии, такие как скринридеры, используют эту информацию, чтобы узнать, что находится на веб-странице и сообщают своим пользователям и это позволяет им взаимодействовать со страницей. Инспектор доступности также использует эту информацию для обеспечения возможностей для отладки доступности в DevTools.
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:
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.
The enabled accessibility panel looks like so:
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:
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.textContent, whereas form elements' names are the contents of their associated {{htmlelement("label")}}.On the right-hand side, you can see further information about the currently selected item. The listed properties are as follows:
<a> element's href.
accessKey attribute. Note that this works correctly as of Firefox 62 ({{bug("1467381")}}).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.
The Accessibility tab is fully keyboard-accessible:
You can print the contents of the accessibility tree to json by right-clicking on an entry in the Accessibility tab and selecting Print to json:
When you do, you will get a new tab with the selected accessibility tree loaded into the JSON viewer:
Once opened, you can save or copy the data as necessary. The JSON viewer can also show you the raw json data on a separate tab in the viewer.
When the accessibility features are turned on, there are a number of useful additional features available in the DevTools, which are detailed below:
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:
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.
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. The role and name of the item will be shown in a small information bar. This is useful for determining how the items in the accessibility tree relate to the UI items on the actual page.
In the following example, you can see that the image has been highlighted and its role, graphic, and name, "Road, Asphalt, Sky, Clouds, Fall" appears in the information bar above it.
Contrast ratio information is particularly useful when you are designing the color palette for your website because if the contrast is not sufficient, readers with visual impairments such as color blindness will be unable to read the text. The Web Content Accessibility Guidelines (WCAG) 2.0 defines 4.5:1 as the minimum suggested contrast ratio between the foreground and background colors for smaller text on a web page. For example:
The color contrast in the image above is 2.77, potentially not enough contrast to make it easy to read. Notice the warning symbol that indicates that the contrast fails to meet the acceptable contrast ratio. Compare that to the following:
In this example, the contrast is 12.63. This time the number is followed by AAA and a checkmark in green, indicating that the text has a contrast ratio of 7:1 or more, meaning it meets the criteria for enhanced contrast, or Level AAA.
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:
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.
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.
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).
