Traditionally, a Doctype, or Document Type Declaration associates the document with a Document Type Definition.
The Document Type Definition is a standard for a specific XML or SGML document. XML and SGML themselves doesn't have much of a schema or a very specific set of rules aside from how tags and attributes work in general. You can think of a DTD a description of the rules for a particular kind of document (like HTML, SVG or MathML). They say what tags are allowed where (e.g. that an html element must contain exactly one head element followed by exactly one body element).
There are alternatives to DTDs such as XML Schemas that are more commonly used today.
Browsers, however, do not use DTDs at all. They read the Doctype to determine the rendering mode, but the rules for parsing the document are entirely baked into the browser.
This is why HTML 5 has a Doctype (to determine the rendering mode) but not DTD.
Rendering Modes
Early web browsers were very buggy. When new versions were released they had to maintain compatibility with their predecessors and rivals. This made it very hard to fix bugs because websites were built that depended on them.
To resolve this, modern browsers have different rendering modes (standards mode, for rendering your document and css according to standards, and quirks mode, wherein the browser emulates the bugs of earlier browsers, and almost standards mode which sits between the two).
Choosing a Doctype
There are two factors to consider when selecting a Doctype:
- Does it trigger standards mode? (For new pages it should, times when you need to be compatible with browsers which don't support standards mode are very rare today).
- Does it support the features I need?
Generally this means you should use HTML 5. It is the current standard and best reflects how browsers actually work:
<!DOCTYPE html>
Failing that. Strict doctypes avoid most features that should be handled with CSS.
When writing in XHTML 1.0, this Doctype is common:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
More obsolete features are available via:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
When writing in HTML 4.01, this one is common instead:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
With the obsolete features being in
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
and
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
"http://www.w3.org/TR/html4/frameset.dtd">
Note that most of the above have variations (e.g. you can omit the URL and rely on the public identifier) which have implications for the support of standards mode. This article includes an extensive list.
Debate on Strict versus Transitional Doctypes
(Note that the following is much, must less true in 2021 than it was in 2008)
Standards evangelists have called for web developers to stop using the Transitional Doctype on new pages and instead use Strict. Again, this is a case where the theory and the practice have some difficulties being reconciled. The original hope of the transitional Doctype was to provide a halfway house for transitioning legacy websites toward standards-compliance. With transitional doctypes, the restriction on elements and attributes is literally "less strict", so developers would be able to get their work running under standards mode sooner, and phase out the outstanding differences over time.
Controversy exists because it isn't always quite so simple for a developer change the Doctype in an enterprise environment. Freelance developers and makers of small- or medium- sized websites may often have an easier time determining their Doctype and making this transition. In an enterprise production environment for a highly-demanded web-based service, there are inherently more complicated dependencies on legacy systems and 3rd party code products, which themselves may be on a roadmap for removal or redesign, but the execution of such changes must be done methodically and incrementally.
Helpful Tools
The W3C (World Wide Web Consortium) is a group which plays an active role in defining these kinds of standards. They maintain a helpful online tool at http://validator.w3.org/ for verifying and validating documents against their standards. There are many other 3rd party tools and browser extensions with similar functionality.
