[DiapDealer]

When you add a blank html file, the xml declaration is automatically created. Unless you explicitly delete it, there's no reason for a new, blank html file NOT to have an xml declaration.

If you have Mend on Save and Open enabled in your preferences, there's no way that I know of to lose "everything." There's really no good reason to disable Mend. It will easily/painlessly fix things like missing xml declarations. It can literally save you from yourself. Disable it at your own peril.

If Mend was enabled, then I've no idea how "everything" could be gone. Even with a crash. It doesn't make sense. Deleting the xml declaration (and continuing to edit) doesn't cause a crash on my system. It just tells me I have xhtml that's not well-formed and gives me the option to fix it automatically, or fix it manually.