In most CMS systeem Today, it is easy to make a website responsive, but since there are so many different devices to choose from today, it often needs to be customized a little extra, lest the design break up on certain screen sizes.
Therefore, it is an advantage that the CMS system the website is built in provides the ability to embed some code, such as HTML, CSS and Javascript.
Then it is possible to enter Viewports for those screen widths that are not covered by the CMS system.
A Viewport defines which code should apply to a screen width ranging from so and so many pixels to so and so many pixels, e.g. from 1600px to 1200px. For example, the menu at the top of the web page can be displayed with the tabs/menu buttons next to each other down to a certain width, until there is no longer room for all the tabs next to each other, and the design breaks.
Then you can either change the font size, for example, or change the menu to become a dropdown menu. In order for these changes to apply only from where the design breaks and on narrower screens, but not in the width span where it can be displayed in full width, one can enter a viewport that applies only to screen widths that are narrower.