XHTML - DOCTYPES
The XHTML standard defines three Document Type Definitions. The most commonly used and easy one is the XHTML Transitional document.
XHTML 1.0 document type definitions correspond to three DTDs:
There are few XHTML elements and attributes which are available in one DTD but not available in another DTD. Therefore, while writing your XHTML document, you must select your XHTML elements or attributes carefully. However, XHTML validator helps you to identify valid and invalid elements and attributes.
Please check XHTML Validations for more detail on this.
XHTML 1.0 Strict
If you are planning to use Cascading Style Sheet (CSS) strictly and avoiding to write most of the XHTML attributes, then it is recommended to use this DTD. A document conforming to this DTD is of the best quality.
If you want to use XHTML 1.0 Strict DTD then you need to include the following line at the top of your XHTML document.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
XHTML 1.0 Transitional
If you are planning to use many XHTML attributes as well as few Cascading Style Sheet properties, then you should adopt this DTD and you should write your XHTML document accordingly.
If you want to use XHTML 1.0 Transitional DTD, then you need to include the following line at the top of your XHTML document.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
XHTML 1.0 Frameset
You can use this when you want to use HTML Frames to partition the browser window into two or more frames.
If you want to use XHTML 1.0 Frameset DTD, then you need to include following line at the top of your XHTML document.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
Note : No matter what DTD you are using to write your XHTML document, if it is a valid XHTML document; then your document will be considered as a good quality document.