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The Essential Elements in an HTML5 document

We will start with the head element and do more later.

The Essential Elements in an HTML5 document

doctype

The HTML syntax requires a doctype to be specified to ensure that the browser renders the page in standards mode. The doctype has no other purpose.
The doctype declaration for the HTML syntax is <!DOCTYPE html> and is case-insensitive. Doctypes from earlier versions of HTML were longer because the HTML language was SGML-based and therefore required a reference to a DTD. This is no longer the case and the doctype is only needed to enable standards mode for documents written using the HTML syntax. Browsers already do this for <!DOCTYPE html>.
Source: http://www.w3.org/TR/html5-diff/#doctype

charset="utf-8"

Unicode Transformation Format 8-bit is a variable-width encoding that can represent every character in the Unicode character set. It was designed for backward compatibility with ASCII and to avoid the complications of endianness and byte order marks in UTF-16 and UTF-32.
Source: http://www.utf-8.com/

title

Title is the only part of the head element that is visible in the browser. It appears on the title bar or the tab depending on which browser is being used.