I have already answered your support ticket on the subject, for convenience I will paste my answer here as well:
he experienced behavior is caused by the fact the 'xmlns' attribute is not XHTML compliant. You can use the RadEditor to strip Word-formatting on paste. You might have a look at this blog post and this demo concerning the PasteFromWord functionality.
If you want to keep the content this way, a possible workaround is to disable the ConvertCharactersToEntities filter. You can test the described behavior with disabled filter in our demo and find more information for the content filters of RadEditor in this article.
I hope you will find this information helpful.
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