CSS stands for Cascading Style Sheet. CSS is used to style HTML code and CSS3 is the latest version to be approved by the World Wide Web Consortium (W3C). While it is possible to style yoru pages directly within HTML, using CSS allows you to separate content from style.
Looking at all of the different font options available can be overwhelming.
HTML makes UGLY pages!! How can you arrange the content as a presentation, not just a group of text?
CSS3 allows you to easily add animation to your site? Simple snippets of code can create cool effects.
If you use media queries you can modify the look and feel of your page based on the platform.
Find out great ways to use colors in your page that will be consistent across machines AND allow people with vision impairment to still navigate your page.
There are a number of popular frameworks that can provide templates for your site. However, these templates can always be improved to meet your specific needs. Knowing CSS allows you to make these changes.