There are three Document Type Definitions (DTDs) available for use with HTML documents. But what is a DTD?
We have seen that HTML is a SGML application, that is, it conforms to the rules set out by the SGML standard. Within that standard, we must define which elements and attributes are allowed. The DTD allows us to do this.
The Strict DTD contains all elements and attributes that have not been deprecated, or made obsolete.
The Transitional DTD includes everything in the Strict DTD, plus all deprecated elements and attributes.
The Frameset DTD has everything in the Transitional DTD as well as attributes and element required to support frames.
Formally, we should declare which DTD a HTML document will use. This will assist the browser when rendering the page.
<!DOCTYPE HTML
PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
"http://www.w3.org/TR/html4/frameset.dtd"
<HTML>
<HEAD>
...
The sample HTML above would tell the browser that the document is based on the Frameset DTD and therefore frames will be used.
