Bootstrap can help you to build your sites faster offerring ready-to-use HTML/CSS code for different layout, buttons, forms. I got used to use such CSS-frameworks to save my time while building standard HTML layouts for my sites.
Bootstrap is a toolkit from Twitter designed to kickstart development of webapps and sites. It includes base CSS and HTML for typography, forms, buttons, tables, grids, navigation, and more.
Bootstrap is built with Less and was designed to work out of the gate with only modern browsers in mind.
Find W3C recommendations here - http://www.w3.org/TR/CSS2/