The wayback bachine is used routinely, at least in Australia, to show that a web document was publicly available before the priority date of an application. One problem is that if the document happens to be from the domain of the applicant (more common than you'd think, some applicants are dumb like that), they can have it removed by archive.org. This is why examiners will make copies of the page from a date before the priority date when using documents accessed through the wayback machine.