Online Editor Documentation, on it's section concerning http://doc.ez.no/Extensions/eZ-Publish-extensions/Online-Editor/5.x/Usage/The-toolbar/The-toolbar/The-Image-and-Object-buttons The "Image" and "Object" buttons briefly mentions the existence of *class* attribute but there are no details, in doc.ez.no pages, as to how the values for these classes are definable.

By inspecting ezoe's content.ini settings one realizes that there is a *embed-type_images* group where *AvailableClasses* can be defined.
The list of AvailableClasses defined in this group is responsible for the options presented in the //edit <embed> tag// popup.

What is not mentioned in this ini file (and in no other document, to my knowledge), is that although selectable, a custom class also needs to be included in [embed]AvailableClasses[] so that the value is not stripped when the XML Block containing the embed tag is saved.

Documentation could be enhanced to make this design restriction evident.