If you’re anything like me you’ve probably added a lot of CSS to your site over the years without ‘pruning’ back the bits that no longer exist. While this isn’t the end of the world, it means that your CSS file continues to grow and send un-needed styles down the pipes. This explains how you can find those unused elements and lighten the load.
A monolithic style sheet is difficult to reason about, which is challenging as CSS files tend to attract bloat. Removing unused styles can help make the situation more manageable.An excerpt from Remove Unused CSS Rules