The HTML 5 language has a "custom" HTML syntax that is compatible with HTML 4 and XHTML1 documents published on the Web, but is not compatible with the more esoteric SGML features of HTML 4.

HTML 5 does not have the same syntax rules as XHTML where we needed lower case tag names, quoting our attributes,an attribute had to have a value and to close all empty elements.

But HTML5 is coming with lots of flexibility and would support the followings −

Uppercase tag names.
Quotes are optional for attributes.
Attribute values are optional.
Closing empty elements are optional.
The DOCTYPE

DOCTYPEs in older versions of HTML were longer because the HTML language was SGML based and therefore required a reference to a DTD.

HTML 5 authors would use simple syntax to specify DOCTYPE as follows −

<!DOCTYPE html>

All the above syntax is case-insensitive.

Character Encoding

HTML 5 authors can use simple syntax to specify Character Encoding as follows −

<meta charset="UTF-8">

All the above syntax is case-insensitive.

The <script> tag

It's common practice to add a type attribute with a value of "text/javascript" to script elements as follows −

<script type="text/javascript" src="scriptfile.js"></script>

HTML 5 removes extra information required and you can use simply following syntax −

<script src="scriptfile.js"></script>

The <link> tag

So far you were writing <link> as follows −

<link rel="stylesheet" type="text/css" href="stylefile.css">

HTML 5 removes extra information required and you can use simply following syntax −

<link rel="stylesheet" href="stylefile.css">