Porting from https://www.w3.org/Bugs/Public/show_bug.cgi?id=25798.

We need web platform tests, and potentially a spec change, to see how many fetches get made in the following scenario:

• An image is fetched, but comes back broken (network error etc.)
• After that fetch completes, another <img> for the same URL is inserted into the document

Do browsers try to fetch again? Or do they just immediately give back the broken image?