In Word for the web, watermarks cannot be created or edited directly. They will display in the document when viewed in View mode, but in Edit mode, they appear as placeholders that cannot be moved or resized. This means that while you can see the watermark in the desktop version after adding it there, it will not show in the web version as a fully functional watermark. The only way to add a watermark in Word for the web is to use the desktop version to create it, as you have already done, but it will not be editable or visible in the web version as intended.
If you need to add a visual element that functions similarly to a watermark, you might consider inserting a picture in the header as you mentioned. However, this method has limitations, as the image will only appear in the header and cannot be expanded to cover the entire page.
For more advanced features like watermarking, the desktop version of Word is recommended since Word for the web has limited capabilities in this regard.
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