Head First HTML with CSS

We can’t have your pages putting
the browser into Quirks Mode!
We’ll all be better off for telling the browser up front: “Hey, we’re
an HTML page that gets it. We’re standards compliant. This is
HTML 4.01, baby!”
When you do that, the browser knows exactly how to handle your
page and (at least on any browser you’d care about) the page is
going to display as you’d expect.
So, how do you tell the browser? Easy, you just add one line to the
top of your HTML files. Here’s what the line looks like:
Okay, we know that is one butt ugly line, but keep in mind, it is
written for your browser, not you. This line is called a document type
definition because it tells the browser the type of the document, and in
this case, the document is your HTML page. Let’s just take a quick
peek at this line to get a feel for it. But again, this is browser speak,
not something you need to know well or memorize. Just throw it in
the top of your HTML and you’re ready to go.
Tells the browser this is
specifying a document
standards, compliance, and all that jazz
This means
that
is the root
(first) element
in your page.
type for this page.You can type this all on
Notice that this is NOT an HTML
element. It has a “!” after the “