The W3C Web Content Accessibility Guidelines 1.0, Checkpoint 6.3, require that you ensure that pages are usable when scripts, applets, or other programmatic objects are turned off or not supported. If this is not possible, provide equivalent information on an alternative accessible page. [This is a Priority 1 requirement]

Making JavaScript accessible involves looking at the following issues (from WEBAIM: Creating Accessible Javascript):

• When using event handlers, use only those that are device independent (e.g., do not require the use of the mouse only)
• Content and functionality that is provided through scripting must be made accessible to assistive technologies (devices and software used to access web content)
• Web pages that utilize scripting must be fully navigable using a keyboard
• JavaScript should not modify or override normal browser functionality in a way that may cause confusion
• When JavaScript cannot be made natively accessible, an accessible alternative must be provided

Javascript can also be optimised for use on the web