Yeah, you also have to account for box sizing by adding borders. What I've done for this in the past is use a css &:after pseudo-element and make it absolutely positioned, and set width and height at 100% and position it at left: 0 and top: 0. You can do things like give it a fill with opacity too, to highlight not with just outline but color overlay too.

.outlined:after { content: ""; position: absolute; top: 0; width: 0; height: 100%; width: 100%; background-color: green; opacity: 0.3; }

.outlined { position: relative; } // this part my be a poor assumption