HTML5 is the new latest and fifth reversion of HTML and it is not w3c recommended because it is being in developing but this the future scope and trend, It offers new features that provide not only rich media support, also enhance support for creating web applications that can interact with the user, many advanced elements and tags and innovative options are there.
This simple introduction, HTML [HyperText Markup Language] is the main markup language for creating web pages and other information that can be displayed in a web browser. Html is written with HTML form elements and angle brackets (like ).
DOCTYPE declaration is must in older versions, in as per HTML5 view it not a mandatory and even it compressed (Reduced). Should have to write DOCTYPE code before HTML tag and starting of the document.
*It's not HTML tag
There is a reason behind of that, In HTML5 only one doctype declaration and it is simple and easy to write.
Here we have example code of HTML5 doctype with HTML and header and body tags.
HTML5 Browser compatibility
HTML5 Not W3c Recommended
HTML5 Not officially released
DOCTYPE defines which version of (X)HTML your document is actually using
DOCTYPE defines which language is using( "Transitional//EN")