Problem

Server-side rendering a hero image can cause double download of the same image when using srcset but not sizes. The problem is: the amp-img extension in that case generates a viewport specific sizes attribute which might cause a re-download of the same image.

Steps to reproduce

Load https://lava-useful-horesradish.glitch.me/ in DevTools simulating a mobile device, e.g. G4. You can see that the image is downloaded twice:

Suggested fix

Don't generate a sizes attribute for ssr'd img. There's no value in setting this anyway, as the user agent has already decided at this point in time which image to download.

//cc @patrickkettner @jridgewell