of the control characters excluded from XML, even when using a numeric character reference. An alternative encoding mechanism such as Base64 is needed to represent such characters. Comments Comments may appear anywhere in a document outside other markup. Comments cannot appear before the XML declaration. Comments begin with <!-- and end with -->. For compatibility with SGML, the string "--" (double-hyphen) is not allowed inside comments; this means comments cannot be nested. The ampersand has no special significance within comments, so entity and character references are not recognized as such, and there is no way to represent characters outside the character set of the document encoding. An example of a valid comment: <!--no need to escape <code> & such in comments--> International use XML 1.0 (Fifth Edition) and XML 1.1 support the direct use of almost any Unicode character in element names, attributes, comments, character data, and processing instructions (other than the ones that have special symbolic meaning in XML itself, such as the less-than sign, "<"). The following is a well-formed XML document including Chinese, Armenian and Cyrillic characters: <?xml version="1.0" encoding="UTF-8"?> <俄语 լեզու="ռուսերեն">данные</俄语> Syntactical correctness and error-handling The XML specification defines an XML document as a well-formed text, meaning that it satisfies a list of syntax rules provided in the specification. Some key points in the fairly lengthy list include: The document contains only properly encoded legal Unicode characters. None of the special syntax characters such as < and & appear except when performing their markup-delineation roles. The start-tag, end-tag, and empty-element tag that delimit elements are correctly nested, with 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 !"#$%&'()*+,/;<=>?@[\]^`{|}~, 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. This policy, occasionally referred to as "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. XML's policy in this area has been criticized as a violation of Postel's law ("Be