I have sever years of experience doing that. I'm a Javascript master and I worked with all the most used framework. Angular 1.5, React, Redux, Backbone, Ionic, Meteor, Jquery. Super familiar with responsive design. Html 4 and 5, css3, less/sass/scss and framework as bootstrap, google-material, etc Also I use: Gulp, grunt, babel, Git and Git Flow. Very familiar with Agile methodologies especially Lean and Feature-driven.