Keep in mind this is just one persons thoughts on what should be included in a DTD for a recipe.
Some tags that might be included are:
One consideration is that each tag must make sense. For example, not everyone will understand the difference between <source> and <origin>. My meaning is to have the <source> tag the title of a book or reference to where the recipe was found. The <origin> tag is for what country or ethenticity the recipe comes from.
Some people using XML to store their recipes may want more options and others would be happy to have less. Many recipes would fit into the structure of <title>, <description>, <ingredients> and <instructions>. I think it is important to include any information that someone looking at the recipe may want to know. A difficulty here is that the information may not be available, for example <calories> would take time to figure out if one didn't know the figure. The DTD can offer flexibility so that there are options.