Responsive image style with srcset and without sizes
Define your own breakpoints group with "multipliers" settings in YOURTHEME.breakspoints.yml  (see D8 Docs)
Go to Admin > Configuration > Media > Responsive Image Styles and add a new image style.
When creating the responsive image style, select your breakpoint group created in (1.). When your YAML is correct, you should see a separate detail set for every multiplier (=dppx).
Select type " Select a single image style." and select your corresponding image size. Repeat for every DPI group defined in your breakpoints.yml
In your node's display settings, for your image field, select the responsive image formatter and choose your responsive image style created in (2.)