This CSS Outline Online Test helps you to Boost your Knowledge in PHP. The outline is a property which is used to draw a line around elements outside the borders drawn above the content and they do not take spaces. It also has a property outline-colour which is used to specify the color of an outline. In this, we also have a grooved effect which is used property is used to specify a 3D grooved outline its value depends on the border color value. A CSS property that sets the thickness of an elements outlier is called outline-width.