The Responsive image style provides three components: a responsive image element, the responsive image style configuration entity, and the responsive image field formatter. The configuration entity is consumed by the field formatter and displayed through the responsive image element.
The responsive image style entity contains an array of breakpoints to image style mappings. The available breakpoints are defined by the selected breakpoint groups. Breakpoint groups can be changed anytime; however, the previous mappings will be lost.
The responsive image element prints a picture element with each breakpoint, defining a new source element. The breakpoint's media query value is provided as the media attribute for the element.
For Internet Explorer 9, Drupal 8 ships with the picturefill polyfill. Internet Explorer 9 does not recognize source elements wrapped by a picture element. The polyfill wraps the sources around a video element within the picture element.