Relatively little of this work seems to consider ongoing preservation beyond the initial digitisation. Even less emphasis is placed on the preservation and long term archiving of web pages.
Web pages are a relatively new phenomenon.
Very few of them date before 1993. Initially much of the data on the web was a mirror of traditionally published information. This has changed and an increasing amount of information is only made available in online form. Unless steps are taken to preserve and archive this data, it may eventually be lost. Unlike books and other physical materials, web sites and web pages are prone to changes in content, to move from one host to another or worse, to disappear completely without trace.