URLs contain CSS files of size over 15 KB. The figure of 15 KB is relatively arbitrary – there is no hard and fast rule as to what constitutes a CSS file that is ‘too large’.
Why it’s important
The smaller CSS files are, the faster they are downloaded, resulting in a faster execution by the browser.
How to fix this issue
CSS files can contain unnecessary characters, such as comments, whitespaces, and indentation. In production, these characters can be safely removed, to reduce file size without affecting how the browser processes the styles. This technique is called minification.
CSS Minification is an industry best practice that should be incorporated into your development workflow so that your production website is optimized for performance. Some content management systems like WordPress have many plugins that help to reduce CSS file size in a few clicks.
CSS file size being over 15 KB analysis is not enough to optimize your site for search engines!
Make a full audit to find out and fix your technical SEO in order to improve your SERP results.