Word Web App corrects common mistakes as you type, such as switching letters that are typed in the wrong order (for example, “teh" is changed to “the”). If the web app makes a correction you don’t want, press Ctrl+Z to undo it. Unlike the Word desktop application, Word Web App does not include a way to customize the AutoCorrect options
If you have the Word desktop application, use the Open in Word command to open the document and use AutoCorrect custom options there.
Word Web App works seamlessly with Word and other Office desktop programs, and we update it based on input like yours. For the latest feature announcements, visit the Office Web Apps blog.