Since I often have problems keeping logged in here (something happening on the server?), I would like to be able to do constructive criticisms offline. As it helps the author of the reviewed text to see where changes have been made, I'd like to make sure the changes are marked up in suitable code. Can anyone advise me of a text editor that can track changes to text with code suitable for posts in this forum?
No. This site does not permit product endorsements. All I can recommend is that you get to know the markup language (you can get details from the FAQ), and use a standard text editor offline. It's really not that difficult, especially if you've ever coded HTML "in the raw". The markup language is not HTML, but it's structurally similarl.