!Doctype declaration, required element

The !DOCTYPE declaration is required.

When !DOCTYPE declaration is present, the web page triggers the Standards Compliance mode.
The !DOCTYPE declaration is required, otherwise (when absent) the web page triggers the Quirks Mode.

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

Invalid Syntax
(doctype declaration is absent)
1 2 3 4 5 6 7 8 9	<html> <head> <⁄head> <body> <⁄body> <⁄html>

Video demonstration !DOCTYPE declaration, required element

HTML5 Doctype required, Tutorial

min	video details
00:00	video tutorial: doctype declaration, required in page
00:05	index.php file: doctype declaration is present → valid syntax
00:10	index2.php file: doctype declaration is absent → invalid syntax
00:13	doctype declaration is required in page → index.php file is HTML5 valid (assuming the rest of the markup is also valid)
00:23	HTML5 specifications require a doctype declaration; in HTML5, doctype's purpose is no longer to declare ⁄define the type of the document but to trigger the Rendering Mode, only!