An HTML document with a doctype of XHTML 1.0 Transitional:
The doctype declaration should be the very first thing in an HTML document, before the <html> tag.
The doctype declaration is not an HTML tag; it is an instruction to the web browser about what version of the markup language the page is written in.
The doctype declaration refers to a Document Type Definition (DTD). The DTD specifies the rules for the markup language, so that the browsers render the content correctly.
Look at our table of all HTML/XHTML elements, and which DTD each element appear in.
This DTD contains all HTML elements and attributes, but does NOT INCLUDE presentational or deprecated elements (like font). Framesets are not allowed.
This DTD contains all HTML elements and attributes, INCLUDING presentational and deprecated elements (like font). Framesets are not allowed.
This DTD is equal to HTML 4.01 Transitional, but allows the use of frameset content.
This DTD contains all HTML elements and attributes, but does NOT INCLUDE presentational or deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML.
This DTD contains all HTML elements and attributes, INCLUDING presentational and deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML.
This DTD is equal to XHTML 1.0 Transitional, but allows the use of frameset content.
This DTD is equal to XHTML 1.0 Strict, but allows you to add modules (for example to provide ruby support for East-Asian languages).
Use W3C's Validator to check that you have written a valid HTML / XHTML document
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