Even if the original webpage disappears, you can often use this informationto locate an archived version using the Wayback Machine, a project of theInternet Archive that preserves a record of websites: https://archive.org/web/.
It would be useful to suggest here:
Ideally one's note taking applications would automatically archive web pages to the Internet Archive as you take notes from them. This means that if they should disappear in the future, you'd have recourse to a useful and workable back up.