I copy and paste it onto a word document and save it to keep it safe. On some browsers like say google chrome you can close the tab or the whole browser and then when you want it back you can go to your browser history and in recently closed tabs the tab should be there and when you load it it should have everything you had written there too. I don't like to risk it though because I have had a couple cases before where it hasn't saved and I've lost everything :3
If you are on a phone I guess you could find a notes app and copy what you have written into that. Just make sure it saves :P