Bootstrap is one of the most popular frontend framework for web development. It consist set of pre define CSS style, components and jQuery plugable libraries for faster web page development.
This tool is using bootstrap version 3.3.0
You can select or search top most code snippet of predefined examples. You also can check your own bootstrap custom code in text editor and run script. Bootstrap or jQuery libraries load by default so you no need to load those files or define <body> element.
CSS media query makes web page compitable for multiple screen size. On the right hand side of tool is a resize panel handler. Drag handler and check page layout in different size of screens.