Front-End Web Developers have a more complex and elaborate job than it was 10 years ago. Web design has evolved from table-based to table-less, from fluid layout to fixed-width layout and back to fluid layout. We used to worry above putting everything above the fold and when a scroll button was introduced to the computer mouse it doesn't matter that much anymore. A few years back Flash animations were the norm and now CSS can do a lot of animations.
Screen sizes have also changed dramatically. Before we just have to make sure that our website will look good on a 960px width, the most widely used screen resolution was just a little bit higher than that. A 960px width will give the site ample white space on each side.
What is a responsive web design?
Responsive web design is a design that adheres to the viewport of the user. There is a spectrum of viewport nowadays and it will be cumbersome to have a separate theme for each one of them. Some developers approach it by building two themes; one for mobile and another for desktop. But there are a lot of screen sizes to consider to be supporting just 2 viewports.
The most strategic approach would be to build a website that can adjust its layout depending on the screen size. This approach is easy to maintain having only one theme to code and just use media queries when needed. The site must be coded with responsive design in mind, this way media queries are lessened.
How do you test for responsive web?
While the site is being built, developers have been testing it for responsiveness on a single browser. On Firefox Developer edition you can easily test the responsiveness of your site without resizing your window. Simply hit CTRL + SHIFT + M on your keyboard and the responsive web testing UI will appear. The device can be selected or you can manually adjust the size on the UI. Chrome and Edge also have a way to test.
There are times when you also need to test on an actual device but if your site is properly built it is seldom you need to do it this way but if you do need to there are a lot of services on the web that offers to test on an actual device.