The issue that you mentioned is caused because this is the default behavior of the RadEditor. The RadEditor by default modifies the content of the HTML5 tags in order to keep the support of the old browsers. If you want to prevent this behavior you can try the following:
1) Download the attached files, extract the archive and add the files inside your SitefinityWebApp project
2) Go to your site’s Administration > Settings > Advanced page and open the Controls > ViewMap node. Create a new entry and for the HostType, enter Telerik.Sitefinity.Web.UI.Fields.HtmlField. Then enter the path to your custom template file in the LayoutTemplatePath property.
If you need any additional information on this matter please do not hesitate to ask
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