UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.186 Safari/537.36

Steps to reproduce the problem:
1. Open url https://codepen.io/zipper/pen/vdwVLb
2. Image inside div is incorrectly scaled.
3. Similar behaviour is observed when sizes attribute is used https://codepen.io/zipper/pen/gvJBdp.

What is the expected behavior?
Image should be 400px wide, since it has "object-fit" property set to "scale-down" value.

What went wrong?
Image is up-scaled. Its size is either corresponding to sizes attribute or viewport width (when sizes is omitted). But source image is smaller and because of "object-fit: scale-down" it shouldn't be upscaled.

On the other hand, "object-fit: cover" works (when width and height are explicitly set in CSS), see https://codepen.io/zipper/pen/PQvxwG - the sizes attributes is overridden by object-fit property.

Did this work before? N/A 

Does this work in other browsers? Yes

Chrome version: 64.0.3282.186  Channel: stable
OS Version: 10.0
Flash Version: 

Tested in FireFox and MS Edge, both shows the image in correct size (400px, not up-scaled). 

One possible use case, when this may happen, is when the images are generated on back-end. We may not have large enough file to generate certain size. In that case we just send smaller file with appropriate width descriptor in srcset attribute, but the sizes attribute is not affected. Sizes attribute is not generated by the server, its copy-pasted from static template, since it depends on actual layout of the page (so it is basically written by a developer and doesn't change). This way we can prevent image from being up-scaled yet we can use responsive images and the behaviour can be controlled through CSS.