!Doctype declaration, occurrences

Only 1 DOCTYPE declaration is allowed per HTML5 document

DOCTYPE Examples:

Valid Syntax
(1 doctype declaration present)
1 2 3 4 5 6 7 8 9	<!doctype html> <html> <head> <⁄head> <body> <⁄body> <⁄html>

Invalid Syntax
(2 doctype declarations present)
1 2 3 4 5 6 7 8 9 10	<!doctype html> <!doctype html> <html> <head> <⁄head> <body> <⁄body> <⁄html>