Instead of doing a bad and lossy job of archiving the page myself, I notify† our friendly neighbourhood archivists at the Internet Archive of the page; and they then do the best, most lossless job of preserving the page that they're able, given their cumulative experience.
† http://blog.archive.org/2017/01/25/see-something-save-someth...
As a side-benefit, they also then take care of keeping the archive they've made around and available online in perpetuity, with no additional marginal effort on my part. The same can't be said for something in my own "private collection."