Copyright ©2002 W3C ® ( MIT , INRIA , Keio ), All Rights Reserved. W3C liability , trademark , document use and software licensing rules apply.
This document summarizes the current best practice for using various Internet media types when serving XHTML Family documents to relatively modern user agents - even those that do not yet support XHTML natively. In summary , 'application/xhtml+xml' SHOULD be used for XHTML Family documents, and the use of 'text/html' SHOULD be limited to HTML -compatible XHTML Family documents intended for delivery to user agents that do not explciitly accept 'application/xhtml+xml'. 'application/xml' and 'text/xml' MAY also be used, but whenever appropriate, 'application/xhtml+xml' or 'text/html' SHOULD be used rather than those generic XML media types.
Note that, because of the lack of explicit support for XHTML (and XML in general) in some legacy user agents, only very careful construction of documents can ensure their portability (see Appendix A ). If you do not require the advanced features of XHTML Family markup languages (e.g., XML DOM, XML Validation, extensibility via XHTML Modularization, semantic markup via XHTML+RDFa, Assistive Technology access via the XHTML Role and XHTML Access modules, etc), authors may want to consider using HTML 4.01 [ HTML ] in order to reduce the risk that content will not be portable to legacy user agents. Even in that case authors can help ensure their portability AND ease their eventual migration to the XHTML Family by ensuring their documents are valid [ VALIDATOR ] and by following the guidelines in Appendix A .
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This document is a Note made available by the World Wide Web Consortium (W3C) for your information. Publication of this Note by W3C indicates no endorsement by W3C or the W3C Team, or any W3C Members.
This document has been produced by the W3C XHTML 2 Working Group as part of the HTML Activity . The goals of the XHTML 2 Working Group are discussed in the XHTML 2 Working Group charter . The document represents working group consensus on the usage of Internet media types for various XHTML Family documents. However, this document is not intended to be a normative specification. Instead, it documents a set of recommendations to maximize the interoperability of XHTML documents with regard to Internet media types. This document does not address general issues on media types and namespaces.
Comments on this document may be sent to www-html-editor@w3.org ( archive ). Public discussion on this document may take place on the mailing list www-html@w3.org ( archive ).
XHTML 1.0 [ XHTML1 ] reformulated HTML 4 [ HTML4 ] as an XML application, and Modularization of XHTML [ XHTMLM12N ] provided a means to define XHTML-based markup languages using XHTML modules, collectively called as "XHTML Family". However, due to historical reasons, a recommended way to serve such XHTML Family documents, in particular with regard to Internet media types, was somewhat unclear.
After the publication of [ XHTML1 ], an RFC for XML media types was revised and published as RFC 3023 [ RFC3023 ], and it introduced the '+xml' suffix convention for XML-based media types. The 'application/xhtml+xml' media type [ RFC3236 ] was registered following that convention. Now that there are at least four possibilities on media type labeling for XHTML Family documents - 'text/html', 'application/xhtml+xml', and generic XML media types 'application/xml' and 'text/xml'.
This document summarizes the current best practice for using those various Internet media types for XHTML Family documents.
The key words " MUST ", " MUST NOT ", " REQUIRED ", " SHALL ", " SHALL NOT ", " SHOULD ", " SHOULD NOT ", " RECOMMENDED ", " MAY ", and " OPTIONAL " in this document are to be interpreted as described in RFC 2119 [ RFC2119 ].
http://www.w3.org/1999/xhtml
.
xml:lang
),
but
an
XHTML
Family
document
type
MAY
also
include
elements
and
attributes
from
other
namespaces,
such
as
MathML
[
MathML2
].
This section summarizes which Internet media type SHOULD be used for which XHTML Family document for which purpose.
The 'text/html' media type [ RFC2854 ] is primarily for HTML, not for XHTML. In general, this media type is NOT suitable for XHTML except when the XHTML is carefully constructed (see Appendix A. In particular, 'text/html' is NOT suitable for XHTML Family document types that add elements and attributes from foreign namespaces, such as XHTML+MathML [ XHTML+MathML ].
XHTML documents served as 'text/html' will not be processed as XML [ XML10 ], e.g. well-formedness errors may not be detected by user agents. Also be aware that HTML rules will be applied for DOM and style sheets (see guidelines 11 and 13 ).
Authors should also be careful about character encoding issues. A typical misunderstanding is that since an XHTML document is an XML document, the character encoding of an XHTML document should be treated as UTF-8 or UTF-16 in the absence of an explicit character encoding information. This is NOT the case when an XHTML document is served as 'text/html'. "6. Charset default rules" of [ RFC2854 ] notes as follows:
The use of an explicit charset parameter is strongly recommended. While [ MIME ] specifies "The default character set, which must be assumed in the absence of a charset parameter, is US-ASCII." [ HTTP ] Section 3.7.1, defines that "media subtypes of the 'text' type are defined to have a default charset value of 'ISO-8859-1'". Section 19.3 of [ HTTP ] gives additional guidelines. Using an explicit charset parameter will help avoid confusion.
Using an explicit charset parameter also takes into account that the overwhelming majority of deployed browsers are set to use something else than 'ISO-8859-1' as the default; the actual default is either a corporate character encoding or character encodings widely deployed in a certain national or regional community. For further considerations, please also see Section 5.2 of [ HTML40 ].
"5.2.2
Specifying
the
character
encoding"
of
the
HTML
4
specification
[
HTML4
]
also
notes
that
user
agents
must
not
assume
any
default
value
for
the
"charset"
parameter
.
Therefore,
authors
SHOULD
NOT
assume
any
default
value
for
an
XHTML
document
served
as
'text/html',
and
as
mentioned
in
[
RFC2854
],
the
use
of
an
explicit
charset
parameter
is
STRONGLY
RECOMMENDED
.
When
it
is
difficult
to
specify
an
explicit
charset
parameter
through
a
higher-level
protocol,
authors
SHOULD
include
the
XML
declaration
(e.g.,
<?xml
version="1.0"
encoding="EUC-JP"?>
)
and
a
meta
http-equiv
statement
(e.g.
<meta
http-equiv="Content-Type"
content="text/html;
charset=EUC-JP" />
).
See
guideline
9
for
details.
The 'application/xhtml+xml' media type [ RFC3236 ] is the primary media type for XHTML Family document types, and in particular it is suitable for all XHTML Host Language document types. XHTML Family document types suitable for this media type include [ XHTML1 ], [ XHTMLBasic ], [ XHTML11 ] and [ XHTML+MathML ]. An XHTML Host Language document type that adds elements and attributes from foreign namespaces MAY identify its profile with the 'profile' optional parameter or other means such as the "Content-features" MIME header described in RFC 2912 [ RFC2912 ]. Each namespace SHOULD be explicitly identified through namespace declaration [ XMLNS ]. This document does not preclude the registration of its own media type for specific XHTML Host Language document type.
In general, this media type is NOT suitable for XHTML Integration Set document types. This document does not define which media type should be used for XHTML Integration Set document types.
'application/xhtml+xml' SHOULD be used for serving XHTML documents to XHTML user agents (agents that explicitly indicate their support for this media type). Authors who wish to support both XHTML and HTML user agents MAY utilize content negotiation by serving carefully constructed XHTML docuemtns both as 'text/html' and as 'application/xhtml+xml'. Alternately, authors may server HTML versions of such documents as 'text/html' and XHTML versions as 'application/xhtml+xml'. Also note that it is not necessary for XHTML documents served as 'application/xhtml+xml' to follow the HTML Compatibility Guidelines .
When serving an XHTML document with this media type, authors SHOULD include the XML stylesheet processing instruction [ XMLstyle ] to associate style sheets.
As for character encoding issues, as mentioned in "6. Charset default rules" of [ RFC3236 ], 'application/xhtml+xml' has the same considerations as 'application/xml'. See section 3.3 for details.
The 'application/xml' media type [ RFC3023 ] is a generic media type for XML documents, and the definition of 'application/xml' does not preclude serving XHTML documents as that media type. Any XHTML Family document MAY be served as 'application/xml'.
However, authors should be aware that such a document may not always be processed as XHTML (e.g. hyperlinks may not be recognized), depending on user agents. Generic XML processors might recognize it as just an XML document which includes elements and attributes from the XHTML namespace (and others), and may not have a priori knowledge what to do with such a document beyond they can do for generic XML documents.
Authors SHOULD explicitly identify the XHTML namespace through the namespace declaration when they serve an XHTML Family document as 'application/xml' to facilitate the chance for reliable processing. The XML stylesheet PI SHOULD be used to associate style sheets.
Whenever appropriate, 'application/xhtml+xml' SHOULD be used rather than 'application/xml'.
As
for
character
encoding
issues,
"3.2
Application/xml
Registration"
of
[
RFC3023
]
says
that
the
use
of
the
charset
parameter
is
STRONGLY
RECOMMENDED
,
and
also
specifies
a
rule
that
[i]f
an
application/xml
entity
is
received
where
the
charset
parameter
is
omitted,
no
information
is
being
provided
about
the
charset
by
the
MIME
Content-Type
header
.
This
means
that
conforming
XML
processors
MUST
follow
the
requirements
described
in
section
4.3.3
of
[
XML10
].
Therefore, while it is STRONGLY RECOMMENDED to specify an explicit charset parameter through a higher-level protocol, authors SHOULD include the XML declaration (e.g. <?xml version="1.0" encoding="EUC-JP"?> ). Note that a meta http-equiv statement will not be recognized by XML processors, and authors SHOULD NOT include such a statement in an XHTML document served as 'application/xml' (and 'application/xhtml+xml' as well for that matter).
The 'text/xml' media type [ RFC3023 ] is an another generic media type for XML documents, and the definition of 'text/xml' does not preclude serving XHTML documents as that media type, either. Any XHTML Family document MAY be served as 'text/xml'. The considerations for 'application/xml' also apply to 'text/xml'. Whenever appropriate, 'application/xhtml+xml' SHOULD be used rather than 'text/xml'.
Authors
should
also
be
aware
of
the
difference
between
'application/xml'
(and
for
that
matter
'application/xhtml+xml'
as
well)
and
'text/xml'
with
regard
to
the
treatment
of
character
encoding.
According
to
"3.1
Text/xml
Registration"
of
[
RFC3023
],
if
a
text/xml
entity
is
received
with
the
charset
parameter
omitted,
MIME
processors
and
XML
processors
MUST
use
the
default
charset
value
of
"us-ascii"[
ASCII
]
.
This
default
value
is
authoritative
over
the
encoding
information
specified
in
the
XML
declaration,
or
the
XML
default
encodings
of
UTF-8
and
UTF-16
when
no
encoding
declaration
is
supplied,
so
omitting
the
charset
parameter
of
a
'text/xml'
entity
might
cause
an
unexpected
result.
As
mentioned
in
[
RFC3023
],
the
use
of
the
charset
parameter
is
STRONGLY
RECOMMENDED
.
The following table summarizes recommendation to content authors for labeling XHTML documents. HTML 4 is also listed for comparison.
Media type | HTML 4 | XHTML Family (HTML compatible) | XHTML Family (other) | XHTML Family + Extensions |
---|---|---|---|---|
text/html | SHOULD | MAY | SHOULD NOT | SHOULD NOT |
application/xhtml+xml | MUST NOT | MAY | SHOULD | SHOULD |
application/xml | MUST NOT | MAY | MAY | MAY |
text/xml | MUST NOT | MAY | MAY | MAY |
This appendix summarizes design guidelines for authors who wish their XHTML documents to render on existing HTML user agents.
New text :
DO NOT include XML processing instructions NOR the XML declaration.
Rationale: Some legacy user agents render XML processing instructions. Also, some user agents interpret the XML declaration to mean that the document is unrecognized XML rather than HTML. Such user agents may not render the document as expected. For compatibility with these types of legacy browsers, you may want to avoid using processing instructions and XML declarations. Remember, however, that when the XML declaration is not included in a document, AND the character encoding is not specified by a higher level protocol such as HTTP, the document can only use the default character encodings UTF-8 or UTF-16.
Original text :
Be aware that processing instructions are rendered on some user agents. Also, some user agents interpret the XML declaration to mean that the document is unrecognized XML rather than HTML, and therefore may not render the document as expected. For compatibility with these types of legacy browsers, you may want to avoid using processing instructions and XML declarations. Remember, however, that when the XML declaration is not included in a document, the document can only use the default character encodings UTF-8 or UTF-16.
New text :
DO
include
a
space
before
the
trailing
/
and
>
of
empty
elements
(e.g.,
<br />
,
<hr />
and
<img
src="karen.jpg"
alt="Karen" />
).
Rationale:
Legacy
user
agents
ignore
the
/>
at
the
end
of
a
tag.
DO
use
the
minimized
tag
syntax
for
empty
elements
(e.g.,
<br
/>
).
Rationale:
Legacy
user
agents
may
give
uncertain
results
when
using
the
as
the
alternative
syntax
<br></br>
allowed
by
XML.
Original text :
Include a space before the trailing
/
and>
of empty elements, e.g.<br />
,<hr />
and<img src="karen.jpg" alt="Karen" />
. Also, use the minimized tag syntax for empty elements, e.g.<br />
, as the alternative syntax<br></br>
allowed by XML gives uncertain results in many existing user agents.
New text :
DO
NOT
use
the
minimized
form
of
elements
whose
content
model
is
not
EMPTY
even
if
the
document
has
no
content
for
the
element
(e.g.,
DO
NOT
express
an
empty
paragraph
as
<p />
).
Rationale : HTML user agents are not aware of the XML-permitted minimization notation.
Original text :
Given an empty instance of an element whose content model is not
EMPTY
(for example, an empty title or paragraph) do not use the minimized form (e.g. use<p> </p>
and not<p />
).
New text :
DO
use
external
style
sheets
if
your
style
sheet
uses
<
or
&
or
]]>
or
--
.
DO
use
external
scripts
if
your
script
uses
<
or
&
or
]]>
or
--
.
Rationale :XML parsers are permitted to silently remove the contents of comments. Therefore, the historical practice of "hiding" scripts and style sheets within "comments" to make the documents backward compatible may not work as expected in XML-based user agents.
Original text :
Use external style sheets if your style sheet uses
<
or&
or]]>
or--
. Use external scripts if your script uses<
or&
or]]>
or--
. Note that XML parsers are permitted to silently remove the contents of comments. Therefore, the historical practice of "hiding" scripts and style sheets within "comments" to make the documents backward compatible is likely to not work as expected in XML-based user agents.
New text :
DO NOT use line breaks and multiple white space characters within attribute values.
Rationale : These are handled inconsistently by user agents.
Original text :
Avoid line breaks and multiple white space characters within attribute values. These are handled inconsistently by user agents.
New text :
DO
NOT
include
more
than
one
isindex
element
in
the
document
head
.
Rationale
:
The
isindex
element
is
deprecated
in
favor
of
the
input
element.
Original text :
Don't include more than one
isindex
element in the documenthead
. Theisindex
element is deprecated in favor of theinput
element.
lang
and
xml:lang
Attributes
New text :
In
XHTML
1.0
documents,
DO
use
both
the
lang
and
xml:lang
attributes
when
specifying
the
language
of
an
element.
The
value
of
the
xml:lang
attribute
takes
precedence.
@@@We
need
a
solution
here
for
XHTML
Family
documents
-
some
way
of
setting
the
language@@@
Rationale
:
When
processing
documents
as
HTML,
user
agents
will
use
the
lang
attribute.
Original text :
Use both the
lang
andxml:lang
attributes when specifying the language of an element. The value of thexml:lang
attribute takes precedence.
New text :
DO
use
the
id
attribute
to
identify
elements.
DO
ensure
that
the
values
used
for
the
id
attribute
are
limited
to
[A-Za-z][A-Za-z0-9:_.-]*
.
Rationale
:
In
HTML
3.2
and
earlier
the
name
attribute
on
some
elements
could
be
used
to
define
an
anchor,
but
HTML
4
introduced
the
id
attribute.
In
an
XML
dialect,
only
attributes
with
type
ID
are
permitted
to
be
used
as
anchors,
and
the
id
attribtue
is
defined
to
be
of
type
ID
.
Relying
upon
the
id
attribute
as
an
anchor
will
work
well
in
legacy
HTML
user
agents
and
XHTML
user
agents.
Original text :
In XML, URI -references [ RFC2396 ] that end with fragment identifiers of the form
"#foo"
do not refer to elements with an attributename="foo"
; rather, they refer to elements with an attribute defined to be of typeID
, e.g., theid
attribute in HTML 4. Many existing HTML clients don't support the use ofID
-type attributes in this way, so identical values may be supplied for both of these attributes to ensure maximum forward and backward compatibility (e.g.,<a id="foo" name="foo">...</a>
).Further, since the set of legal values for attributes of type
ID
is much smaller than for those of typeCDATA
, the type of thename
attribute has been changed toNMTOKEN
. This attribute is constrained such that it can only have the same values as typeID
, or as theName
production in XML 1.0 Section 2.3, production 5. Unfortunately, this constraint cannot be expressed in the XHTML 1.0 DTDs. Because of this change, care must be taken when converting existing HTML documents. The values of these attributes must be unique within the document, valid, and any references to these fragment identifiers (both internal and external) must be updated should the values be changed during conversion.Note that the collection of legal values in XML 1.0 Section 2.3, production 5 is much larger than that permitted to be used in the
ID
andNAME
types defined in HTML 4. When defining fragment identifiers to be backward-compatible, only strings matching the pattern[A-Za-z][A-Za-z0-9:_.-]*
should be used. See Section 6.2 of [ HTML4 ] for more information.Finally, note that XHTML 1.0 has deprecated the
name
attribute of thea
,applet
,form
,frame
,iframe
,img
, andmap
elements, and it will be removed from XHTML in subsequent versions.
New text :
DO set the character encoding for a document via the charset parameter of HTTP Content-Type header.
Rationale : Since these guidelines already recommend that documents NOT contain the XML declaration, setting the encoding via the HTTP header is the only reliable mechanism compatible with legacy user agents.
Original text :
Historically, the character encoding of an HTML document is either specified by a web server via the charset parameter of the HTTP Content-Type header, or via a
meta
element in the document itself. In an XML document, the character encoding of the document is specified on the XML declaration (e.g.,<?xml version="1.0" encoding="EUC-JP"?>
). In order to portably present documents with specific character encodings, the best approach is to ensure that the web server provides the correct headers. If this is not possible, a document that wants to set its character encoding explicitly must include both the XML declaration an encoding declaration and ameta
http-equiv statement (e.g.,<meta http-equiv="Content-type" content="text/html; charset=EUC-JP" />
). In XHTML-conforming user agents, the value of the encoding declaration of the XML declaration takes precedence.Note: be aware that if a document must include the character encoding declaration in a meta http-equiv statement, that document may always be interpreted by HTTP servers and/or user agents as being of the internet media type defined in that statement. If a document is to be served as multiple media types, the HTTP server must be used to set the encoding of the document.
New text :
DO
use
the
full
form
for
boolean
attributes,
as
required
by
XML
(e.g.,
disabled="disabled"
).
Rationale : The compact for for these attributes is not well formed XML, and therefore invalid.
Original text :
Some HTML user agents are unable to interpret boolean attributes when these appear in their full (non-minimized) form, as required by XML 1.0. Note this problem doesn't affect user agents compliant with HTML 4. The following attributes are involved:
compact
,nowrap
,ismap
,declare
,noshade
,checked
,disabled
,readonly
,multiple
,selected
,noresize
,defer
.
Original text :
The Document Object Model level 1 Recommendation [ DOM ] defines document object model interfaces for XML and HTML 4. The HTML 4 document object model specifies that HTML element and attribute names are returned in upper-case. The XML document object model specifies that element and attribute names are returned in the case they are specified. In XHTML 1.0, elements and attributes are specified in lower-case. This apparent difference can be addressed in two ways:
- User agents that access XHTML documents served as Internet media type
text/html
via the DOM can use the HTML DOM, and can rely upon element and attribute names being returned in upper-case from those interfaces.- User agents that access XHTML documents served as Internet media types
text/xml
,application/xml
, orapplication/xhtml+xml
can also use the XML DOM. Elements and attributes will be returned in lower-case. Also, some XHTML elements may or may not appear in the object tree because they are optional in the content model (e.g. thetbody
element withintable
). This occurs because in HTML 4 some elements were permitted to be minimized such that their start and end tags are both omitted (an SGML feature). This is not possible in XML. Rather than require document authors to insert extraneous elements, XHTML has made the elements optional. User agents need to adapt to this accordingly. For further information on this topic, see [ DOM2 ]
New text :
DO
ensure
that
the
reserved
character
&
is
included
in
content
in
its
escaped
form
&
.
Rationale : If ampersands are not encoded, the characters after them up to the next semi-colon can be interpreted as the name of a entity by the user agent.
Original text :
In both SGML and XML, the ampersand character ("&") declares the beginning of an entity reference (e.g., ® for the registered trademark symbol "®"). Unfortunately, many HTML user agents have silently ignored incorrect usage of the ampersand character in HTML documents - treating ampersands that do not look like entity references as literal ampersands. XML-based user agents will not tolerate this incorrect usage, and any document that uses an ampersand incorrectly will not be "valid", and consequently will not conform to this specification. In order to ensure that documents are compatible with historical HTML user agents and XML-based user agents, ampersands used in a document that are to be treated as literal characters must be expressed themselves as an entity reference (e.g. "
&
"). For example, when thehref
attribute of thea
element refers to a CGI script that takes parameters, it must be expressed ashttp://my.site.dom/cgi-bin/myscript.pl?class=guest&name=user
rather than ashttp://my.site.dom/cgi-bin/myscript.pl?class=guest&name=user
.
New text :
DO use lower case element and attribute names in style sheets.
DO
create
rules
that
include
inferred
elements
(e.g.,
the
tbody
element
in
a
table).
DO
explicitly
set
the
style
on
the
html
element,
since
in
XHTML
CSS
rules
the
style
on
the
body
is
not
automatially
reflected
there.
@@@@NEED NEW RATIONALE@@@@
Original text :
The Cascading Style Sheets level 2 Recommendation [ CSS2 ] defines style properties which are applied to the parse tree of the HTML or XML documents. Differences in parsing will produce different visual or aural results, depending on the selectors used. The following hints will reduce this effect for documents which are served without modification as both media types:
- CSS style sheets for XHTML should use lower case element and attribute names.
- In tables, the tbody element will be inferred by the parser of an HTML user agent, but not by the parser of an XML user agent. Therefore you should always explicitly add a tbody element if it is referred to in a CSS selector.
- Within the XHTML namespace, user agents are expected to recognize the "id" attribute as an attribute of type ID. Therefore, style sheets should be able to continue using the shorthand "#" selector syntax even if the user agent does not read the DTD.
- Within the XHTML namespace, user agents are expected to recognize the "class" attribute. Therefore, style sheets should be able to continue using the shorthand "." selector syntax.
- CSS defines different conformance rules for HTML and XML documents; be aware that the HTML rules apply to XHTML documents delivered as HTML and the XML rules apply to XHTML documents delivered as XML.
DO NOT use xml stylesheet declarations to identify style sheets.
DO
use
the
script
or
link
elements
to
define
stylesheets.
Rationale
:
Since
XML
processing
instructions
may
be
rendered
by
some
legacy
user
agents,
using
the
standard
XML
stylesheet
declaration
mechanism
may
not
work
well.
However,
since
XHTML
user
agents
are
required
to
process
style
and
link
elements
and
intpret
stylesheets
referenced
from
those
elements,
documents
constructed
to
use
them
will
work
as
expected.
Original text :
In HTML 4 and XHTML, the
style
element can be used to define document-internal style rules. In XML, an XML stylesheet declaration is used to define style rules. In order to be compatible with this convention,style
elements should have their fragment identifier set using theid
attribute, and an XML stylesheet declaration should reference this fragment. For example:<?xml-stylesheet href="http://www.w3.org/StyleSheets/TR/W3C-REC.css" type="text/css"?> <?xml-stylesheet href="#internalStyle" type="text/css"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <title>An internal stylesheet example</title> <style type="text/css" id="internalStyle"> code { color: green; font-family: monospace; font-weight: bold; } </style> </head> <body> <p> This is text that uses our <code>internal stylesheet</code>. </p> </body> </html>
DO NOT use the formfeed character (U+000C).
Rationale : This character is recognized as white space in HTML 4, but is NOT considered white space in XML.
Original text :
Some characters that are legal in HTML documents, are illegal in XML document. For example, in HTML, the Formfeed character (U+000C) is treated as white space, in XHTML, due to XML's definition of characters, it is illegal.
DO
use
'
to
speficy
an
escaped
apostrophe.
DO
NOT
use
'
.
Rationale
:
The
entity
'
is
not
defined
in
HTML
4.
Original text :
The named character reference
'
(the apostrophe, U+0027) was introduced in XML 1.0 but does not appear in HTML. Authors should therefore use'
instead of'
to work as expected in HTML 4 user agents.
" HTML 4.01 Specification ", W3C Recommendation, D. Raggett, A. Le Hors, I. Jacobs, eds. , 24 December 1999. Available at: http://www.w3.org/TR/1999/REC-html401-19991224
The latest version of HTML 4.01 is available at: http://www.w3.org/TR/html401
The latest version of HTML 4 is available at: http://www.w3.org/TR/html4
" Mathematical Markup Language (MathML) Version 2.0 ", W3C Recommendation, D. Carlisle, P. Ion, R. Miner, N. Poppelier, eds. , 21 February 2001. Available at: http://www.w3.org/TR/2001/REC-MathML2-20010221
The latest version is available at: http://www.w3.org/TR/MathML2
The W3C Markup Validation Service available at http://validator.w3.org.
" XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition): A Reformulation of HTML 4 in XML 1.0 ", W3C Recommendation, S. Pemberton et al. , August 2002. Available at: http://www.w3.org/TR/2002/REC-xhtml1-20020801
The first edition is available at: http://www.w3.org/TR/2000/REC-xhtml1-20000126
The latest version is available at: http://www.w3.org/TR/xhtml1
" XHTML™ 1.1 - Module-based XHTML ", W3C Recommendation, M. Altheim, S. McCarron, eds. , 31 May 2001. Available at: http://www.w3.org/TR/2001/REC-xhtml11-20010531
The latest version is available at: http://www.w3.org/TR/xhtml11
" XHTML™ Basic ", W3C Recemmendation, M. Baker, M. Ishikawa, S. Matsui, P. Stark, T. Wugofski, T. Yamakami, eds. , 19 December 2000. Available at: http://www.w3.org/TR/2000/REC-xhtml-basic-20001219
The latest version is available at: http://www.w3.org/TR/xhtml-basic
" Modularization of XHTML™ ", W3C Recommendation, M. Altheim, F. Boumphrey, S. Dooley, S. McCarron, S. Schnitzenbaumer, T. Wugofski, eds. , 10 April 2001. Available at: http://www.w3.org/TR/2001/REC-xhtml-modularization-20010410
The latest version is at: http://www.w3.org/TR/xhtml-modularization
" Extensible Markup Language (XML) 1.0 Specification (Second Edition) ", T. Bray, J. Paoli, C. M. Sperberg-McQueen, E. Maler, eds. , 6 October 2000. Available at: http://www.w3.org/TR/2000/REC-xml-20001006
The latest version is available at: http://www.w3.org/TR/REC-xml
" Namespaces in XML ", T. Bray, D. Hollander, A. Layman, eds. , 14 January 1999. Available at: http://www.w3.org/TR/1999/REC-xml-names-19990114
The latest version is available at: http://www.w3.org/TR/REC-xml-names
" Associating Style Sheets with XML documents Version 1.0 ", W3C Recommendation, J. Clark, ed. , 29 June 1999. Available at: http://www.w3.org/1999/06/REC-xml-stylesheet-19990629
The latest version is available at: http://www.w3.org/TR/xml-stylesheet
In 3.5. Summary , changed 'text/html' for HTML 4 as SHOULD rather than MAY .
Updated reference to XHTML 1.0 to refer to the Second Edition.