It's subjective, but if there are two words in an element tag, the readibility can be enhanced by adding an underscore between words (e.g. <my_tag>
) instead of using no separator. Reference: http://www.w3schools.com/xml/xml_elements.asp. So according to w3schools the answer would be:
<my_tag attribute="some value">
The value needn't use an underscore or separator, since you are allowed spaces in attribute values but not in element tag names.