That's for the initial resource itself, not subsequent asset requests. For instance, an asset request is a separate request on a different domain, such as one for a customer service analytics tag, which is requested without any accept resource type request filter.

So if the initial request comes with "Accept: text/html" then you serve a webpage. The request when linked as img comes as "Accept: /" and you serve an image.

Perhaps caching or other schemes would break this, I'll try to knock up a PoC.