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 modern browsers in mind.
Bootstrap is tested and supported in major modern browsers like Chrome, Safari, Internet Explorer, and Firefox.
Bootstrap comes complete with compiled CSS, uncompiled, and example templates.