People with low vision or people with cognitive disabilities may benefit from a clear indication of which form control currently has focus. This is largely controlled by the user's browser. However, CSS can be used to provide an additional indicator.
Use the CSS ":focus" pseudo-class to select and style an input when it gains focus. These are well supported in most browsers and easy to apply.
// Using the :focus CSS pseudo class
input:focus
{
background-color:yellow;
}