Frequently Asked Questions

Well-formed XML documents must contain matching start and end tags, and tags cannot overlap. For example, the following example is well-formed:

<?xml version="1.0"?>
<person>
  <name>John Doe</name>
  <occupation>Software Engineer</occupation>
</person>

Whereas, this example is not well-formed (<name> and <occupation> tags overlap, and there is no <person> end tag):

<?xml version="1.0"?>
<person>
  <name>John Doe
  <occupation>Software Engineer
  </name>
  </occupation>

Valid XML documents must conform to a Document Type Definition (DTD). The following example is a valid XML document that conforms to person.dtd:

<?xml version="1.0"?>
<!DOCTYPE PERSON SYSTEM "person.dtd">
<person>
  <name>John Doe</name>
  <occupation>Software Engineer</occupation>
</person>

All valid XML documents are well-formed; however, a well-formed document is not valid unless it conforms to a DTD.

XML in the XML processing instruction (PI), and its attributes, must be in lowercase letters, for example:

<?xml version="1.0"?>

Yes. Unlike in HTML, all attribute values must be quoted in XML documents. the following example shows a correctly quoted certified attribute for the <occupation> element:

<?xml version="1.0"?>
<person>
  <name>John Doe</name>
  <occupation certified="Y">Software Engineer</occupation>
</person>

The following example shows an incorrectly specified certified attribute (not quoted) for the <occupation> element.

<?xml version="1.0"?>
<person>
  <name>John Doe</name>
  <occupation certified=Y>Software Engineer</occupation>
</person>

Attributes may be quoted using single quotes (for example, certified='Y') or double quotes (for example, certified="Y"). Quotes must be used in matching pairs.

No. XML does not allow </> as an end tag. Matching start and end tags must be present, for example:

• <person></person> is a valid start/end tag pair.
• <person></> is not a valid start/end tag pair.

SGML, from which XML is derived, allows </> as a valid end tag.

Yes. If the content model for the element is MIXED -- that is, it contains the #PCDATA keyword -- then the elements can have text interspersed between them.

In the following example, the XML data for the <person> element is valid because the person element contains the #PCDATA keyword:

<?xml version="1.0" ?>
<!DOCTYPE person [
<!ELEMENT person (#PCDATA|lastname|firstname)>
<!ELEMENT lastname (#PCDATA)>
<!ELEMENT firstname (#PCDATA)>
]>
<person>
  My first name is <firstname>John</firstname>
  and my last name is <lastname>Smith</lastname>
</person>