Lighten And Darken With CSS Brightness Filter

CSS has a filter property that can be used with a variety of filter functions. One of them is the brightness() filter. By feeding a percentage less than 100% to brightness(), the target element will be made darker. Inversely, feeding a percentage greater than 100% to brightness() will make the element brighter.

.brighter-span {
  filter: brightness(150%);
}

.darker-span {
  filter: brightness(50%);
}

brighter, regular, and darker spans

See this CSS Tricks Article on the filter property for more details. Check out the browser support story here.