Recommended list of Doctype declarations
The list is informative and does not try to be exhaustive (there are many other proper declarations you could use), but it has most of the declarations commonly used on the Web at the moment.
Recommended Doctype Declarations to use in your Web document.
When authoring document is HTML or XHTML, it is important to Add a Doctype declaration. This makes sure the document will be parsed the same way by different browsers.
The simplest and most reliable doctype declaration to use is the one defined in HTML5 :
If you need a doctype matching a specific version of (X)HTML, the doctype declaration must be exact (both in spelling and in case) to have the desired effect, which makes it sometimes difficult. To ease the work, below is a list of recommended doctype declarations that you can use in your Web documents.
Use the following markup as a template to create a new HTML document using a proper Doctype declaration. See the list below if you wish to use another DTD.
(X)HTML Doctype Declarations List
MathML Doctype Declarations
Compound documents doctype declarations
XHTML + MathML + SVG – DTD. XHTML + MathML + SVG Profile (XHTML as the host language) – DTD. XHTML + MathML + SVG Profile (Using SVG as the host) – DTD.
Optional doctype declarations
Beyond the specificities of (X)HTML processing, Doctype declarations in XML languages are only useful to declare named entities and to facilitate the validation of documents based on DTDs. This means that in many XML languages, doctype declarations are not necessarily useful.
The list below is provided only if you actually need to declare a doctype for these types of documents.
Historical doctype declarations
The doctype declarations below are valid, but have mostly an historical value — a doctype declaration of a more recent equivalent ought to be used in their stead.