I had quite the fright last night when I started getting server errors with WR (wordreference.com). I use it every day for German and have the German and Spanish dictionaries bookmarked (pairs with English). I just figured out that they apparently changed the url from wordreference.com/de to like wordreference.com/deen, and the similarly for Spanish. So all I had to do was adjust my bookmarks to reflect that. Since I didn't have the main site bookmarked, I didn't notice until I thought to back up the url in my address bar to just the main url. While I doubt such a source would disappear, others we often use may. My general rule is to "get all you can get while you can get it." Thus for any smaller sites I save pages that I would hate to lose. I use the Scrapbook addon for Firefox to do this. I have done this with a lot of HTLAL pages, though unfortunately there is no way to save an entire thread in one go with Scrapbook. For smaller sites, I have sometimes just spidered the site and saved the entire thing with a different program. There is a thread on HTLAL by user "random review" where he bemoans the loss of a Memrise deck online, but which he had actually saved (he just prefers the Memrise interface). So such user generated content can also disappear (here apparently due to copyright concerns). Some sites can later be found archived by Internet Archive and one can pull up versions from previous years. But if a site owner lets the domain expire, often a placeholder page with a "no follow" tag will be put up, which prevents IA from storing the older versions. But you have to know the url first, since currently there is no way to search IA. So my advice again regarding internet sources is to get all you can get while you can get it.