C9

You should actually avoid doing this using CSS or JavaScript. I will explain to you why.

When the browser starts loading a page, it starts preloading all the images before the loading of the main parser and the interpretation of the page’s CSS and JavaScript. In general, that mechanism seems useful to reduce page-loading times, but it does not help with the responsive images. Hence, you have to create a solution in this case like ‘srcset’. For example, you could not load the ‘img’ element, then detect the viewport using JavaScript, and then make the dynamic changes to the source image to a smaller one if needed. By then, the original image would have already been loaded, and you will attempt to load the small image as well, which is even destructive in terms of a responsive image.