Bootstrap

Bootstrap is a very popular HTML, CSS, and JavaScript framework made for mobile-first frontend web development.

It was invented at Twitter, by Mark Otto and Jacob Thornton, and released in 2011. With its many HTML and CSS-based design templates, it focuses on simplifying and accelerating the development of informative web pages.

Bootstrap is tested in and compatible with the major modern browsers (Safari, Google Chrome, Firefox, etc). It is very easy to set up, and does not require in-depth knowledge of HTML or CSS.

Additionally, it is fully customisable, and has a large community composed of its users.