Hmm let me see
onLoad - now wouldn't that report a success in loading any page? Though curiously it does not wait for rendering to finish (in my not so humble experience).
I am always amazed when I see the word "NO!"
I believe in the words "Try this" - if they don't produce results it has made the tryer think and look and think again. This is a community after all.
"NO!" never made anything.
It's a matter of how one puts a question. A well structured, clear and correct question will bring a well structured, clear and correct answer. Should have the OP made it clear from the beginning that he wants to know how works the Working Offline feature, the answer would not have been NO
Regarding the onload, I must tell you that, even in theory onload could be applied on all the tags, in practice it works selectively, according to the browser.