If it is not too fast, it would be useful to see the progress, if even just a #/# indicator of the listed files being cached. This should follow nicely from the "progress" events emitted by the applicationCache. Another idea could be a progress bar. By the time requests are sent, the manifest has been parsed so we know the total number of resources, and requests that should be made. It may be that in practice this happens so fast that it isn't worth showing a UI for. But, if downloads are serially (which they appear to be now in WebKit) that would be a better than nothing way to help track down which resource caused an error.
FYI: Chrome does this by way of console logging.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.
This is still a valid enhancement.
<rdar://problem/19281593>