Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive, mobile first content. Bootstrap is an open source toolkit for developing with HTML5, CSS3, and JS. It allows quick prototyping of ideas or you can build your entire app with their Sass variables and mixins, responsive grid system, extensive prebuilt components, and powerful plugins built on jQuery.
To see a sample of a “1 page” Bootstrap website, click on the image above. This is called a “1 page” website because all the content is on the one page and can be accessed with a finger (or thumb) swipe. It comes in handy with a cell phone or pad.