Bootstrap is the most popular HTML, CSS, and JS framework for developing responsive
When you've been using HTML for many years, you know as well as I think there are things that you always do when you start a brand new project and start working with it, you may want to make your input Fields because the ones the browser does not show is so great you want to make some flutter buttons, popup windows and more as you always spend some time at the start of a project to make.
This is where Bootstrap comes into the picture. If you use Bootstrap, you can start by creating your website, administration panel quickly and you can always follow customize your design that you want to get very easy. The good thing about Bootstrap is that they have done it Responsive so you can quickly work with desktop, mobile and tablet devices without having to write tons of CSS, however, there are some things in Bootstrap's responsive that does not quite make you do not have to go into your own CSS code to customize some items to the simple Unity's dissolving resolution.
But by using Bootstrap you will find that your work with HTML and CSS becomes easier when you need to start a new project from scratch again, or need to customize a design and change small items you want to change design.