CSS stands for Cascading Style Sheets.
CSS is a special style to format the HTML pages to be more interesting that can not be done by the HTML tags into it.
CSS has been standardized by the W3C, so it can run on all major browsers.

What advantages css ?

1. With CSS, you will be able to shorten your work time to create Web pages.
2. HTML file size will be smaller, because the CSS files located outside the HTML (if using the External Stylesheet).

What lack of css?
Not all browsers interpret the CSS code in the same way. So sometimes, with CSS web interface looks good in one browser, but the mess in other browsers. So you should check the display to look good in all browsers and add a special code specific browser if necessary in order to display your website looks good in all browsers.

But don’t worry, modern browsers are capable of handling well css syntax.