BOOTSTRAP is an Open Source Front-End framework developed by Twitter that is used for making web development easier and faster. This is NOTHING but a reusable code that we can freely download and use for our own web development so that we get the functionality without having to re-write the same code (i.e., not inventing the wheel again). It supports ALMOST all of the browsers and hence can WORK with any browser.

Bootstrap also makes the website responsive i.e., they fit the screen no matter which device is being used to view the website.

Advantages of Bootstrap over CSS:

• Time-Saving: It increases the development speed as we can achieve the same functionality by using the bootstrap in our code instead of writing the code from scratch.
• Cross-Browser COMPATIBILITY: As bootstrap supports all of the major modern web browsers, we don't need to worry about whether a functionality runs on all modern web browsers or not.
• Responsiveness: As bootstrap uses a fluid grid layout, it adapts to the screen resolution of the device and hence we won't get a surprising website view when opening the same website on different devices of different sizes like a laptop, desktop, mobile, tablet etc
• Ease of Use: With just basic knowledge of HTML and CSS, we can start using bootstrap easily.
• Open Source: Bootstrap is available for free and hence we don't need to pay to get this functionality.