Source: Hypertext Markup Language - 2.0 (RFC 1866: text / html)
Source: Internationalization of the Hypertext Markup Language (RFC 2070: text / html)
Source: HTML 3.2 Reference Specification (W3C Recommendation)
Source: HTML 4.0 Specification (W3C Recommendation)
Strict DTD (text / html)
Transitional DTD (text / html)
Frameset DTD (text / html)
Warning: HTML 4.0 Specification has been superceded by HTML 4.01 Specification.
Source: HTML 4.01 Specification (W3C Recommendation)
Strict DTD (text / html)
Transitional DTD (text / html)
Frameset DTD (text / html)
Warning: ISO/IEC 15445 is still work in progress. It is not yet an International Standard and it is not appropriate to cite it other than as work in progress.
Source: HTML 4.0 Guidelines for Mobile Access (W3C Note)
Mobile DTD (text / html)
Strict DTD (text / html)
Preparation DTD (text / html)
Note: This is a complete subset of HTML 4.0 Strict and ISO-HTML, provided for helping to create contents suitable for mobile access. This DTD has three modes by switching marked sections; that is, a complete subset of HTML 4.0 Strict and ISO-HTML, a fully compatible definition with HTML 4.0 Strict DTD as amended by the HTML 4.0 Specification Errata, and a mode for enforcing the strict heading order like Pre-HTML of ISO-HTML.
Warning: This DTD is provided only for helping to creating contents. Contents conforming to this DTD should be labeled as HTML 4.0 Strict when you publish the contents.
Source: Compact HTML for Small Information Appliances (W3C Note)
Note:
I think the definition of META element is an error.
The following modifications might be necessary.
<!ELEMENT HEAD O O (%head.content) +(META)>
<!ELEMENT TITLE - - (#PCDATA)* -(META)>
<!ELEMENT META - O EMPTY>
<!ATTLIST META
http-equiv NAME #IMPLIED
content CDATA #REQUIRED
>