The mentioned attributes are not related to the HTML5 functionality. I assume they are some third-party JS widget features used for accordion creation.
Nevertheless, testing such content in a simple RadEditor control (not in SharePoint) the content is correctly preserved even with the ConvertToXhtml
filter enabled. You can review this screencast
for this matter.
Note that SharePoint has built-in content filters that clean, strip or modify the saved content. Unfortunately the Telerik Editor WebParts cannot control these filters and I cannot suggest you a proper approach to find a workaround to use the third-party widget in content saved in SharePoint.
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