I’m wondering if anyone can answer ‘why’ to this one.
According to the livedocs on Macromedia’s section of Dreamweaver, there must be a DOCTYPE declaration in the document prior to the root element, and the declaration must reference one of the three Document Type definition (DTD) files for XHTML (strict, transitional, or frameset).
What it doesn’t say is why.
Nor does the W3 consortium say anything about it, other than that it must be in the page. No real (or clear) reason why.
I have tried publishing pages without this declaration and have had no problem.