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DTD (Document Type Definition)
An XHTML DTD describes the allowed syntax and grammar of XHTML markup. Every XHTML document must start with a DTD declaration and a line of code that declares that you are starting to write XHTML code.
- A DTD specifies the syntax of a web page in SGML
- DTDs are used by SGML applications, such as HTML, to specify rules for documents of a particular type, including a set of elements and entity declarations
- An XHTML DTD describes in precise, computer-readable language, the allowed syntax of XHTML markup
There are three XHTML DTDs:
- STRICT
- TRANSITIONAL
- FRAMESET
XHTML 1.0 Strict
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
Use the strict DOCTYPE when you want really clean markup, free of presentational clutter. Use it together with CSS.
XHTML 1.0 Transitional
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Use the transitional DOCTYPE when you want to still use HTML's presentational features.
XHTML 1.0 Frameset
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
Use the frameset DOCTYPE when you want to use HTML frames.
