CSS :focus Selector
Select and style an input field when it gets focus:
More "Try it Yourself" examples below.
Definition and Usage
:focus selector is used to select the element that has focus.
Tip: The :focus selector is allowed on elements that accept keyboard events or other user inputs.
The numbers in the table specifies the first browser version that fully supports the selector.
Note: For :focus to work in IE8, a <!DOCTYPE> must be declared.
When an <input type="text"> gets focus, gradually change the width from 100px to 250px:
transition: ease-in-out, width .35s ease-in-out;
CSS tutorial: CSS Pseudo classes