Accessibility: Physical Impairments
Not all users have the same physical capabilities. Some users cannot use a mouse and must use modified keyboard arrangements to utilize a computer system.
When using or designing a system, it is interesting and important to test it with keyboard navigation in mind. Without any additional changes to settings, most webpages can be operated with just a keyboard using the Tab key to navigate between interactable elements and Space or Enter to select the object.
Let's experiment with the following elements. Select the first text box, enter data if you would like, and try tabbing through the objects and using the form without your mouse.
Note that the tab order does not allow the user to navigate to the second element in the form and skips straight to the Send button. While this may only be a mild irritant to a user who uses mouse and keyboard for navigation, consider a user with one functional hand who is only able to use a keyboard for navigation — they are unable to access this element.
It is vital that proper tab orders are set for elements on a page, or else a user who is limited to keyboard navigation cannot navigate the page appropriately — a violation of accessibility.