Proper HTML formatting
Indent tags and tag contents in parent/child relationships--for example, if you have a list have <ul></ul> at same indent, then <li></li> indented more than parent, and then the content between <li></li> indented more. Ditto all other parent/child tag relationships----e,g, the text between <P> and </P> is indented more than the P tags
Line breaks should produce a clean left/right margin as well
I have yet to see an HTML editor of any kind that produces clear, easy to read markup that is separate from content.