The Expected Output value is the HTML that you expect the page to output. This is empty by default.
This is available so you can compare the current output with the previous output. It's typically used for unit tests and a way for you to show changes between versions from Web Export.
It basically creates diff patches that are viewable as a web page. If you use version control, it is like that but it allows you to see changes from your XD project.
When you use this and there are a lot of changes it can take time to process all of the differences. It may look like the export process has stopped responding. Nay. It is simply comparing differences. It can take up to a minute.
If the HTML is different than the expected output a diff button will show. Click on this button to see the HTML of all the code differences. Use the Enter key to go through each change.
If there are no changes then a checkmark icon will be shown.
To use this copy the page HTML and paste it into this field. The next time you press export a comparison will be made.