Design Reponsive web application

To Design Reponsive web application
 1) Add media query in CSS
Display 24 px in h2, but 16px h2 in mobile CSS
h2{
      font-size: 24px;
}
add media query
@media screen and (max-width: 39.9375em){
    h2{
          font-size: 16px;
    }
} 
2) Usefule softwares and tools 
A) Install npm and node.js - Javascript  package manager
npm opens up an entire world of JavaScript talent to you and to your team. It's the world's largest software registry, with approximately 3 billion downloads per week.The registry contains over 600,000 packages (building blocks of code). Open-source developers from every continent use npm to share and borrow packages.

Install Node.js & npm

If you're using OS X or Windows, use one of the installers from the Node.js download page. Be sure to install the version labeled LTS. Other versions have not yet been tested with npm.

more about install npm
https://docs.npmjs.com/getting-started/installing-node

node.js download:
https://nodejs.org/en/

B) Bootstrap (can be downloaded from https://getbootstrap.com/)

Build responsive, mobile-first projects on the web with the world's most popular front-end component library.

Bootstrap is an open source toolkit for developing with HTML, CSS, and JS

C) AngularJS (can be downloaded from https://angular.io/guide/quickstart)

AngularJS is a JavaScript framework. It can be added to an HTML page with a <script> tag.
AngularJS extends HTML attributes with Directives, and binds data to HTML with Expressions.