For most web text I heartily agree with your quote. But in some cases, e.g., a large family of pages which are to share nontrivial layout (perhaps one page documenting each function in a software library, with standardized formatting for the arglist, the preconditions and postconditions, one or more usage examples...) it might be appropriate to get the shared organization/layout decisions straight before trying to compose much of the text.
Why/How would anyone write CSS before HTML?
1: http://www.brainyquote.com/quotes/quotes/c/charlesbab141832....