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An XML attribute can only have a single value and each attribute can appear at most once on each element.
In the common situation where a list of multiple values is desired, this must be done by encoding the list into a well-formed XML attribute XML documents consist entirely of characters from the Unicode repertoire.
The specification places requirements on what an XML processor must do and not do, but the application is outside its scope.
The processor (as the specification calls it) is often referred to colloquially as an XML parser.
XML provides escape facilities for including characters that are problematic to include directly.
For example: All permitted Unicode characters may be represented with a numeric character reference.
XML-based formats have become the default for many office-productivity tools, including Microsoft Office (Office Open XML), Open and Libre Office (Open Document), and Apple's i Work. Apple has an implementation of a registry based on XML. Many of these standards are quite complex and it is not uncommon for a specification to comprise several thousand pages.
XML has also provided the base language for communication protocols such as XMPP. In publishing, DITA is an XML industry data standard.
Generally, strings that constitute markup either begin with the character An element is a logical document component that either begins with a start-tag and ends with a matching end-tag or consists only of an empty-element tag.
extends the set of allowed characters to include all the above, plus the remaining characters in the range U 0001–U 001F.
At the same time, however, it restricts the use of C0 and C1 control characters other than U 0009 (Horizontal Tab), U 000A (Line Feed), U 000D (Carriage Return), and U 0085 (Next Line) by requiring them to be written in escaped form (for example U 0001 must be written as or its equivalent).
The Unicode character set can be encoded into bytes for storage or transmission in a variety of different ways, called "encodings".
Unicode itself defines encodings that cover the entire repertoire; well-known ones include UTF-8 and UTF-16.