Hi anonymous user ,
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Does this issue occur on other computer?
And what browser are you using? To further confirm if the problem is with the browser itself, please try try to use other browsers(such Microsoft Edge) and log in to OWA to check if the HTML email is displayed properly.
If it is displayed properly on other browsers, it's suggested that you could uninstall and reinstall the browser you current use to check if there're any differences.
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