By rotation do you mean rotation of the page within the document (for use in other viewers as well), or do you mean just a visual rotation within WebViewer?
If by rotation you mean a rotation of the document page itself, unfortunately, this is not supported for any files other than PDFs.
If you mean visual rotation this is supported and can be done through the header bar in UI to rotate all pages.
The UI to rotate the individual pages is coming in the next release (soon) but it can still be done in the code using the Core API.
Here is a code snippet and some relevant APIs