As you may already know, you had the ability to have a preview of a quarantine email before deciding what to do. But this preview was basically raw data (plain view), meaning if the email was in HTML format you basically got the HTML code displayed, making it complicated to review it.
An update has been deployed which allows you to choose if you want to view the raw data (as before) or switch to HTML view.
This applies to both end-user self service quarantine (accessible using https://admin.protection.outlook.com/quarantine) or for administrators reviewing quarantine emails from the Security and Compliance portal (https://protection.office.com/#/quarantine)
When previewing a quarantine email, just switch to HTML view (end-user) or HTML (administrators)
NOTE end-user will get a notification to remind them even if the content has been sanitized, this still could be dangerous