Why is it that CSS cannot be written in an HTML document and vice versa?

Answer 1

it can be, even in the Head of the document with the <style>p{font-weight: Bold;}</style> even though its easyer to maintain when you put it in an external file

Answer 2

Well, css can be embeded in Html. HTMl in css wouldn't be logical, because without the html, the css doesn't know what to do.