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If you are using the Bootstrap grid framework then the following tool could be helpful: https://kalamuna.github.io/bootstrap-picture-srcset-generator/
When given Given an aspect rationratio, this tool generates picture/source markup based on how many grid units wide you want your image to be at any given breakpoint. This markup can be copied directly into html templates or alternatively you can use the image widths and heights generated at each breakpoint to inform the creation of Drupal image styles and responsive images.
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