DTD files are called document type definition files. In DTD files are text files that contain, among other things, element types, element attributes, and notations, and specify their order and nesting. A DTD file defines the structure of other XML, SGL, HTML or XHTML documents.
Usually DTD files in a web page are referenced via a URL, so often the xhtml1-transitional.Dtd or xhtml1-strict.Dtd is included via DOCTYPE statement. Furthermore, DTD files are also used to validate documents. For example, XML documents can be checked for correct structure and element properties to ensure that they can be read correctly.
DTD files can be opened and edited in any text editor, but can only be used with an XML editor or other programs that can parse XML, XHTML, HTML or SGL.
Most often resolving problems with opening DTD files is very simple. Just install an appropriate program that supports such files. All of the listed programs support DTD files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of DTD files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate DTD files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the XMLSpy applications is not on the list, select "Browse" option in order to manually locate the directory where XMLSpy has been installed.
Sometimes problems with opening DTD files may not lie with the application itself, but can arise due to other causes, such as: