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In forms containing many controls, the user may become confused when trying to determine where they are.
Offer the user a clear means of quickly determining what control in the form is currently active, or has received focus.
Complex forms can easily overwhelm the user. Any affordance that helps the user keep track of where they are in the process of completing a form is valuable. Causing the active form control to have a visual indication that it has received focus is a simple and effective means to help accomplish this goal.
The form control that has received focus may have a specific style (CSS) applied, altering its background color, label, or border.