many doctype use a url link 

like this 

```
  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

```

and this dtd file is on live url <http://www.w3.org/TR/html4/strict.dtd> 

What is the use of this online live dtd and how any page (which use this doctype) will render properly according to this doctype without having access to this url (i mean if internet access is not available?)

**update :** I found this info from wikipedia <http://en.wikipedia.org/wiki/System_identifier>

> 
> In HTML and XML, a system identifier
>  is a fragmentless URI reference. It
>  typically occurs in a Document Type
>  Declaration. In this context, it is
>  intended to identify a document type
>  which is used exclusively in one
>  application, whereas a public
>  identifier is meant to identify a
>  document type that may span more than
>  one application.
> 
> 
> In the following example, the system
>  identifier is the text contained
>  within quotes:
> 
> 
> 
> 

**update 2 :** is it only to use for Validators? how some software like dreamweaver provides offline validation?

**update 3:** i found this info from w3c site <http://www.w3.org/QA/Tips/Doctype>

> 
> Why specify a doctype? Because it
>  defines which version of (X)HTML your
>  document is actually using **(version
>  for what browser or validator?)**, and
>  this is a critical piece of
>  information needed by some tools
>  **(which tools? any other tools then validator?)** processing the document.
> 
> 
> For example, specifying the doctype of
>  your document allows you to use tools
>  such as the Markup Validator to check
>  the syntax of your (X)HTML. Such tools
>  won't be able to work if they do not
>  know what kind of document you are
>  using.
> 
> 
> But the most important thing is that
>  with most families of browsers, a
>  doctype declaration will make a lot of
>  guessing unnecessary, and will thus
>  trigger a "standard" rendering mode.
> 
> 
>