If you use iCloud and your HTML email signature appears blank or keeps disappearing after you have followed our Apple Mail Mavericks 10.9 instruction guide, you will need to do the following:
- Go to System Preferences > iCloud
- Uncheck "documents & data"
This is a known issue with Apple Mail, you can read more about it here - https://discussions.apple.com/message/24418496#24418496 . It looks like Apple are aware of the issue and working on it.