20%, do we just make numbers up?
It is technically 20%, because a bunch of the requests that happen on page load do succeed. Just not the few crucial ones that are required for the page to load correctly - those return a 500 the vast supermajority of the time.
For supermajority: the construction means “beyond majority” or “above majority”, it is implemented as a higher percentage in most systems, and there is a utility in being able to refer to high ratio majority.
So people have repurposed a technical word to a general word, of roughly the same meaning.
"Disappoint" was originally the inverse of "appoint" and meant to remove from office.