CSS is one a few ways used to style blank HTML or XML.
It receives so many updates year upon year that new features are generally quite small on the grand scale of things. This means that websites can have new instructions that are passed to the browser to make the site appear fancy.
But why use CSS?
Prior to CSS, sites were styled (and can still be styled) as follows:
Hello there <font text="#f00"> This is red text </font>
Now what happens when a piece of text needs to be in the same paragraph as the red text, but also wants to add bold to it? It then needs nested again - a
<font> element within one another.
This will get messier the more fonts there are.
CSS solves this problem completely and also makes it easier to write and maintain.
If the site has a site wide theme, like this one does, CSS brings the benefit of 'one change does it all' to the site. It also allows the styling information to be cached, resulting in smaller files to be transported from the web server hosting. Such an example is when this website is first loaded, it take a little longer than when another page on the site is loaded after.
This is why CSS is used.