An XML document is valid if its contents match against a Document Type Declaration (DTD) or a XML Schema. This implies that the document must have a correct syntax and all the constraints defined in the schema are satisfied.
Well Formed XML Documents
An XML document is well-formed if its syntax is valid, i.e. if it satisfies the following rules:
- XML documents must have a root element
- XML elements must have a closing tag
- XML tags are case sensitive
- XML elements must be properly nested
- XML attribute values must be quoted
<?xml version="1.0" encoding="UTF-8"?>
<mail>
<to>Alice</to>
<from>Bob</from>
<subject>Wow!</subject>
<body>Don't forget me this weekend!</body>
</mail>XML Documents with Errors
Errors in XML documents will stop your XML applications.
W3C specification states that a program should stop processing an XML document if it finds an error.
Valid vs Well Formed XML Document
A well formed XML document is different from a valid XML document.
A valid XML document must be well-formed and must conform to a Document Type Definition (DTD) or to a XML Schema
Learn more with our XML DTD Tutorial and XML Schema Tutorial