Either write it entirely in a text editor first or copy it into a text editor and save from time to time (ideally under different file names for creating a revision history and redundancy). Some online forms such as Discourse forum software supports draft saving. Then, this is less necessary. But if you spent more than 5 minutes writing anything, backing it up in an offline text file is recommended. Alternatively, one can save the page as HTML, which should include form contents. In early 2014, I lost text of a few kilobytes of text in a WinRAR archive comment (I can't believe I used that **proprietary** closed-source crap back then. Although WinRAR is very functional, proprietary = vendor lock-in risk.), because I cancelled the archive file creation after realizing that I missed some parameters. But that caused the unfinished file with the *archive comment* inside to be erased. I thought of that a few seconds too late. I should have written it in a text editor instead. In 2019, I created backups of my Wikipedia draft articles (two with 20+ KB), but far too rarely (once every few weeks), because I was not aware of Wikipedia's [deletionism extremism](https://en.everybodywiki.com/Wikipedia#Deletionism). I should have backed it up **every single day**.