Event handlers accompany existing HTML code or dynamically generated content and are triggered by a browser or user event - such as when the page loads, when the user clicks the mouse, or when the time is 8 p.m. Some event handlers are dependent upon use of a mouse or keyboard. These are called device dependent event handlers. Other event handlers are device independent and are triggered by both the mouse and keyboard or by other means.
To ensure accessibility, use either a device independent event handler (one that works with both the mouse and the keyboard) or use both mouse dependent and keyboard dependent event handlers.
onmousever event handler (and related
mouseenter events) is triggered when the mouse cursor is placed over an item. As its name implies,
onmousever requires the use of a mouse, thus it is a device dependent event handler and may introduce accessibility issues.
onmousever, and its companion,
onmouseout (and related
mouseleave events), may be used, as long as any important content or functionality is also available without using the mouse. If the mouse interaction is purely cosmetic (such as the addition of a glow or drop shadow), there are likely no accessibility issues, so long as the style change does not indicate some function (such as to indicate that an element is clickable).
If the mouse interaction presents additional information or content, such as a tooltip, a navigation menu, etc., then this content will not be accessible to anyone not using a mouse. For screen reader users (who very often use a keyboard for navigation), the additional content might be provided directly in an accessible way, such as via alternative text, through an ARIA label or description, or perhaps through off-screen text. However, for sighted keyboard-only users, there must be a mechanism for them to access and view the newly revealed content or functionality.
Sometimes scripting is used to present complex mouse interactions, such as a drop-down or fly-out menu. While these can be made technically keyboard accessible, sometimes an accessible alternative approach may be more friendly. For example, instead of forcing users to navigate through a complex and lengthy navigation menu, you could instead ensure that the sub-menu items are NOT directly keyboard accessible (nor read by a screen reader), but standard links provide functionality on the top-level menu item (e.g., "Products"). This link would take the user to a secondary page that provides standard links to the pages provided through the complex menu (e.g., a Products landing page that has links to the various product categories). While not exactly the same interaction that mouse users may choose, such alternatives are often more intuitive and friendly for all users.
One might use
focusin events) and
focusout events) when the keyboard is used to navigate to and from an element. These event handlers are typically used with form inputs, such as text fields, radio buttons, and submit buttons, but can also be used with links or perhaps ARIA widgets.
onfocus is triggered when the cursor is placed on or within a specific form element, or when a user 'tabs' to the element using the keyboard.
onblur is triggered when the cursor leaves a form element or the user 'tabs' away from it.
Both of these event handlers are device independent, meaning that they can be performed with the mouse, keyboard, or other assistive technology. The actions that are performed as a result of these event handlers must be analyzed to determine if they cause accessibility problems, especially if they are modifying the default behavior of the web browser or are interfering with keyboard navigation within the web page. Examples of such issues include automatically setting focus to other page areas after
onblur are triggered, trapping the user inside a form control, dynamically revealing form controls immediately upon a user leaving (blurring) a form input, etc.
onclick event handler (and
click event) is triggered when the mouse is pressed and released when over a page element or when the Enter key is pressed while a keyboard-navigable element has focus. In these cases,
onclick is a device independent event handler.
However, the Enter key will not always trigger
onclick if it is used with non-link and non-control elements, such as plain text, table cells, or
<div> elements, even if they made keyboard navigable using
tabindex or are focused using scripting. In these cases, it will be necessary to detect the Enter and Space key presses while focus is placed on them. Using standard HTML controls often avoids the necessity of scripting and potential accessibility issues.
ondblclick event handler (and
dblclick event) is associated with the double click of a pointing device (typically a mouse or trackpad) on a selected element. Similarly,
mousedown events are only available via pointing devices. These should typically be avoided.
onchange event handler (and
change event) is triggered when a form input value is changed, for example, when a radio button is selected, when the text changes within a text box or text area, or when the active item in a select menu changes. Although these events are device independent and can be activated using the mouse, keyboard, or other device, the actions that are performed as a result of these events must be analyzed to determine if they cause accessibility problems.
A common use of
onchange is on select menus to trigger navigation when the active option within the menu is changed. In some browsers these menus can cause keyboard accessibility issues because the user cannot scroll through the list using a keyboard without selecting one of the options, and thus triggering the
change event. Removing
onchange and providing a submit button separate from the list of choices that activates the currently selected item provides a more accessible and user-friendly alternative.
Using Device Independent Event Handlers
Several event handlers are device independent, including
onclick is used with a keyboard-focusable element). When possible, use device independent event handlers. Other event handlers are device dependent, meaning that they rely wholly upon a certain type of input. For instance,
ondblclick rely upon the use of a pointer device. There are also some event handlers that are dependent upon use of the keyboard (