Table of contents

Appendices

1.3 Rationale and list of changes for XML 1.1

The W3C's XML 1.0 Recommendation was first issued in 1998, and despite the issuance of many errata culminating in a Third Edition of 2004, has remained (by intention) unchanged with respect to what is well-formed XML and what is not. This stability has been extremely useful for interoperability. However, the Unicode Standard on which XML 1.0 relies for character specifications has not remained static, evolving from version 2.0 to version 4.0 and beyond. Characters not present in Unicode 2.0 may already be used in XML 1.0 character data. However, they are not allowed in XML names such as element type names, attribute names, enumerated attribute values, processing instruction targets, and so on. In addition, some characters that should have been permitted in XML names were not, due to oversights and inconsistencies in Unicode 2.0.

The overall philosophy of names has changed since XML 1.0. Whereas XML 1.0 provided a rigid definition of names, wherein everything that was not permitted was forbidden, XML 1.1 names are designed so that everything that is not forbidden (for a specific reason) is permitted. Since Unicode will continue to grow past version 4.0, further changes to XML can be avoided by allowing almost any character, including those not yet assigned, in names.

In addition, XML 1.0 attempts to adapt to the line-end conventions of various modern operating systems, but discriminates against the conventions used on IBM and IBM-compatible mainframes. As a result, XML documents on mainframes are not plain text files according to the local conventions. XML 1.0 documents generated on mainframes must either violate the local line-end conventions, or employ otherwise unnecessary translation phases before parsing and after generation. Allowing straightforward interoperability is particularly important when data stores are shared between mainframe and non-mainframe systems (as opposed to being copied from one to the other). Therefore XML 1.1 adds NEL (#x85) to the list of line-end characters. For completeness, the Unicode line separator character, #x2028, is also supported.

Finally, there is considerable demand to define a standard representation of arbitrary Unicode characters in XML documents. Therefore, XML 1.1 allows the use of character references to the control characters #x1 through #x1F, most of which are forbidden in XML 1.0. For reasons of robustness, however, these characters still cannot be used directly in documents. In order to improve the robustness of character encoding detection, the additional control characters #x7F through #x9F, which were freely allowed in XML 1.0 documents, now must also appear only as character references. (Whitespace characters are of course exempt.) The minor sacrifice of backward compatibility is considered not significant. Due to potential problems with APIs, #x0 is still forbidden both directly and as a character reference.

Finally, XML 1.1 defines a set of constraints called "full normalization" on XML documents, which document creators SHOULD adhere to, and document processors SHOULD verify. Using fully normalized documents ensures that identity comparisons of names, attribute values, and character content can be made correctly by simple binary comparison of Unicode strings.

A new XML version, rather than a set of errata to XML 1.0, is being created because the changes affect the definition of well-formed documents. XML 1.0 processors must continue to reject documents that contain new characters in XML names, new line-end conventions, and references to control characters. The distinction between XML 1.0 and XML 1.1 documents is indicated by the version number information in the XML declaration at the start of each document.