What is that <!DOCTYPE html> at the beginning of the HTML5 document?
This alerts the reader of the document (typically the browser) of the specific type of
HTML document to follow. There are a few different HTML standards. The <!DOCTYPE html>
directive tells the browser that the remainder of the document conforms to the
HTML5 standard Document Type Definition. Lets back up a second. HTML documents
are (now) actually XML documents that conform to particular, well known
Document Type Definitions.
There are a few options for 'html' documents
<!DOCTYPE html>
for html5
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
for HMTL 4.01 transitional
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
for HTML 4.01, but allow framesets
...
Basically the difference between these is the set of tags
the browser is willing to accept in a valid document. By accept I mean,
understands and can properly render as the author would hope. Operationally,
when the browser sees the directive, it could select a particular rendering
engine to read the rest of the document.
Block level elements (note: XMP tag has been deprecated, use PRE tag instead and explicitly escape < >