Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The World Wide Web Consortium's XML 1.0 Specification of 1998 and several other related specifications-all of them free open standards—define XML documents may begin with an XML declaration that describes some information about themselves. An example is .. Many developers choose to use 4-space or 2-space indentation. In XML, each nested statement (e.g., a statement following a '{' brace) should be indented exactly once more than the previous line's indentation.)