Generally edit-on Pro supports arbitrary DTDs for validating XHTML contents as follows.

XHTML 1.0 Transitional DTD

edit-on Pro is fully XHTML 1.0 Transitional compliant as the applet validates contents against the the official W3C XHTML 1.0 Transitional DTD.

Custom DTD

edit-on Pro allows you to use custom DTDs in order to limit the set of elements and attributes in your documents or in order to add custom attributes and elements. However, this will only have an effect on the content that is imported into the editor, i.e. when the content is imported, the editor will parse it and remove any attributes or elements not allowed according to the specified DTD.

XHTML 1.0 Strict DTD

edit-on Pro is NOT XHTML 1.0 Strict compliant out-of-the-box, but it is possible to achieve this compliance with some customization. Unfortunately, using another DTD than a subset of the XHTML 1.0 Transitional DTD or an extended version of this DTD is not recommended at the moment, as this will cause problems when cut and paste or drag and drop of content is used, because of the mechanism the editor uses to reinsert the content. Actually, when cut and paste or drag an drop is used, a document fragment will be exported to the clipboard, and will again be validated when it is imported. Now this is what causes problems, as XHTML 1.0 Strict for example does not allow textual content inside the body element.

For more information about using DTDs please refer to the integration manual (chapter 7, Using Custom DTDs) and the online Custom DTD Sample