Editing XML (section)

== Syntactical correctness and error-handling ==
{{Main|Well-formed document}}

The XML specification defines an XML document as a [[Well-formed element|well-formed]] text, meaning that it satisfies a list of syntax rules provided in the specification. Some key points include:
* The document contains only properly encoded legal Unicode characters.
* None of the special syntax characters such as <code>&lt;</code> and <code>&amp;</code> appear except when performing their markup-delineation roles.
* The start-tag, end-tag, and empty-element tag that delimit elements are correctly nested, with [[tag omission|none missing]] and none overlapping.
* Tag names are case-sensitive; the start-tag and end-tag must match exactly.
* Tag names cannot contain any of the characters <code>!"#$%&amp;'()*+,/;&lt;=&gt;?@[\]^`{|}~</code>, nor a space character, and cannot begin with "-", ".", or a numeric digit.
* A single root element contains all the other elements.

The definition of an XML document excludes texts that contain violations of well-formedness rules; they are simply not XML. An XML processor that encounters such a violation is required to report such errors and to cease normal processing.<ref name="quovadis">{{cite conference|first=Maik|last=Stührenberg|date=February 2013|title=Quo vadis XML? History and possible future directions of the Extensible Markup Language|conference=XML Prague 2013|location=Prague|isbn=978-80-260-3872-6|pages=141–162|conference-url=https://archive.xmlprague.cz/2012/files/xmlprague-2012-proceedings.pdf}}</ref><ref name="bray 2005">{{cite journal|first=Jim|last=Gray|date=1 February 2005|title=A Conversation with Tim Bray: Searching for ways to tame the world's vast stores of information|journal=Queue|volume=3|number=1|pages=20–25|doi=10.1145/1046931.1046941|doi-access=free|s2cid=23502115}}</ref> This policy, occasionally referred to as "[[Draco (lawgiver)|draconian]] error handling", stands in notable contrast to the behavior of programs that process [[HTML]], which are designed to produce a reasonable result even in the presence of severe markup errors.<ref>{{cite book|first=Mark|last=Pilgrim|date=6 August 2010|title=HTML5: Up and Running: Dive into the Future of Web Development|url=https://books.google.com/books?id=Mk3sW0on7OAC|publisher=O'Reilly|isbn=978-1-4493-9966-5|page=10}}</ref> XML's policy in this area has been criticized as a violation of [[Postel's law]] ("Be conservative in what you send; be liberal in what you accept").<ref>{{cite web|url=http://diveintomark.org/archives/2004/01/08/postels-law|archive-url=https://web.archive.org/web/20110514120305/http://diveintomark.org/archives/2004/01/08/postels-law|archive-date=2011-05-14|title=There are No Exceptions to Postel's Law [dive into mark&#93;|website=DiveIntoMark.org|access-date=22 April 2013}}</ref>

The XML specification defines a [[XML validation|valid XML document]] as a [[well-formed XML document]] which also conforms to the rules of a [[Document Type Definition]] (DTD).{{sfnp|Harold|Means|2002|p=29}}