The document type declaration is required for your theme. This is certainly one of the easiest steps in creating a theme!
The theme engine loads the core/doctype.php file in your theme directory and expects to find the theme_page_doctype hook defined. Your hook should return the doctype string.
The following returns the old 4.01 transitional doctype.
function theme_page_doctype() { return "<!DOCTYPE HTML PUBLIC '-//W3C//DTD HTML 4.01 Transitional//EN'>\n"; }
A more modern HTML 5 example:
function theme_page_doctype() { return "<!DOCTYPE html>\n"; }
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