Navigation list item hover & focus effect

Creating a custom hover and focus effect for navigation items is a fairly straightforward task, leveraging the power of CSS transformations.
For each element in your navigation list, you can use the ::before pseudo-element to create a hover effect. By default, you can hide it using transform: scale(0). Then, using the :hover and :focus pseudo-class selectors, you can transition the pseudo-element to transform: scale(1) and show its colored background.
Finally, prevent the pseudo-element from covering the anchor element using z-index.

li a {
  position: relative;
  display: block;
  z-index: 0;
}

li a::before {
  position: absolute;
  content: "";
  width: 100%;
  height: 100%;
  bottom: 0;
  left: 0;
  background-color: #2683f6;
  z-index: -1;
  transform: scale(0);
  transition: transform 0.5s ease-in-out;
}

li a:hover::before,
li a:focus::before {
  transform: scale(1);
}

Finally, prevent the pseudo-element from covering the anchor element using z-index.

From 30-seconds-of-code: navigation-list-item-hover-and-focus-effect