1. Srcset and sizes attributes

    The `srcset` and `sizes` attributes on `img` (or `source`) elements allow authors to define various image resources and "hints" that assist a user agent to determine the most appropriate image source to display (e.g. high-resolution displays, small monitors, etc).

  2. html element: img: attributionsrc

  3. html element: img: src

  4. headers http header: content-security-policy: img-src