Declaring that the document contains HTML5 mark-up with the doctype html5
The doctype for HTML5 is very simple. To indicate that your HTML content uses HTML5, simply use:
Doing so will cause even browsers that don’t presently support HTML5 to enter into standards mode, which means that they’ll interpret the long-established parts of HTML in an HTML5-compliant way while ignoring the new features of HTML5 they don’t support.
This is much simpler than the former doctype html5, and shorter, making it easier to remember and reducing the amount of bytes that must be downloaded.