A framework is a collection of reusable code and common design patterns for websites. No need to start from scratch.
Bootstrap comes with HTML and CSS to get your projects started. There are so many examples to follow in their documents.
Hello, Responsive design!

Responsive design is presenting different layouts of a website in proportion to a device's screen size. This ability to adapt to different screen sizes is used with CSS Media Queries.
stage 0 - Hello, World Wide Webstage 1 - Let's learn HTMLstage 2 - Beautiful pages with CSSstage 3 - Sketch your ideastage 4 - Bootstrap for speedy design- stage 5 - Build your site