XHTML

A Strictly Conforming XHTML Document is an XML document that requires only the facilities described as mandatory in this specification. Such a document must meet all of the following criteria:

1- It must conform to the constraints expressed in one of the three DTDs found in DTDs and the following elements have prohibitions on which elements they can contain (see SGML Exclusions). This prohibition applies to all depths of nesting, i.e. it contains all the descendant elements.

a
must not contain other a elements.
pre
must not contain the img, object, big, small, sub, or sup elements.
button
must not contain the input, select, textarea, label, button, form, fieldset, iframe or isindex elements.
label
must not contain other label elements.
form
must not contain other form elements.

2- The root element of the document must be html.

3- The root element of the document must contain an xmlns declaration for the XHTML namespace [XMLNS]. The namespace for XHTML is defined to be http://www.w3.org/1999/xhtml. An example root element might look like:

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">

4- There must be a DOCTYPE declaration in the document prior to the root element. The public identifier included in the DOCTYPE declaration must reference one of the three DTDs found in DTDs using the respective Formal Public Identifier. The system identifier may be changed to reflect local system conventions.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<!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">

5- The DTD subset must not be used to override any parameter entities in the DTD.

Faq