Although SLDs are the default method to apply styles to the data served by GeoServer, it is not the only way. There is an extension available that adds the ability to provide styling information using Cascaded Style Sheets (CSS). There are a couple of reasons why you might choose to use CSS styles over traditional SLDs:
If you have built any kind of web page, then you are likely already familiar with the syntax of CSS
The CSS syntax is much leaner than SLD, making styles more readable
You don't need to understand XML, making CSS styles less prone to syntax errors
Getting started with CSS styles is as easy as installing the extension and restarting GeoServer.