All of these doctypes may be used to create HTML documents:
All are useful and functional.
ISO believes ISO HTML to have a life of up to 25 years.
* not official releases, but easily created. The XHTML Legacy DTD is a combination of Transitional and Frameset, XHTML Minimal can be created by IGNOREing the Tables, Forms and Images modules in XHTML Basic. Both are safely deliverable as 'text/html'.