HTML and CSS3 have many options to create great designs, but, start from scratch or build designs for all devices, is difficult and requires a great effort. Twitter Bootstrap gives a large list of utilities and components that helps to create high quality designs.
Twitter Bootstrap is written in lesscss, but it can be used directly as CSS3 and there is also a SASS version available. Currently there are many pages and apps using it, and even AngularJS has many tools for its integration.
The best strength is that Twitter Bootstrap can be configured quickly to mimic a corporative image.