differences between html and xhtml

8/4/2019 Differences Between HTML and XHTML

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Differences Between HTML and XHTML

This page is currently being revised. Some information is incomplete or missing.

Please note that the information in here is based upon the current spec for (X)HTML5.Some of the issues technically do not apply to previous versions of HTML.

Although HTML and XHTML appear to have similarities in their syntax, they are

significantly different in many ways.

Note: As the current WHATWG document is a draft, this section will need to

track to a moving target.

Overlap Language

There is a community who find it valuable to be able to serve HTML5 documents whichare also valid XML documents. They may, for example, use XML tools to generate thedocument, and they and others may process the document using XML tools. These

documents are served as text/html.

This language is sometimes called "polyglot". It is the overlap language of documents

which are both HTML5 documents and XML documents. Guidelines are listed below forhow one can construct such a polyglot document which will work in either environment.

Besides following the well-formedness rules of XML, there are some other restrictions to

which one must adhere (for the sake of text/html documents).

This wiki web page is an example of such a document. You can parse it with an XMLparser or an HTML parser.

MIME Types

FeatureHTML

RequirementXHTML Requirement Notes

Mime

Type

Must use

text/html.

Must use an XML MIME type,

such as application/xml or

application/xhtml+xml.

It is the MIME type that

determines what type of

document you are using. Any

document, including adocument authored with the

intention of being XHTML,

served as text/html is

technically an HTML

document.


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purpose

s.

Error

Handling

HTML does not have a well-

formedness constraint, noerrors are fatal. Graceful error

handling and recoveryprocedures are thoroughlydefined.

Well-formedness errors are

fatal

Ensurethere are no

well-formedness

errors.

Charact

er

Encodin

g

(includi

ng XML

Declara

tion,

meta)

The XML declaration is

forbidden (treated as a bogus

comment, but such style ofcomments are deprecated),

but the meta element with a

charset attribute may be

used instead.

If the encoding is unspecified

in HTML, it should be

determined throughimplementation specific

heuristics or fallback to a

default value (Note: thissection of the spec is not yet

finished).

The XML declaration may be

used tospecify the character

encoding, while meta is only

allowed as case-insensitive

"UTF-8" (and is ignored if

included).

The default characterencoding for XHTML is,

according to XML rules, UTF-

8 orUTF-16.

If you need

to include

XML 1.1-

only

markup, if

you do not

wish to

convert the

encoding

of the

document

to UTF-8

or UTF-16

(since use

of other

encodings

also

requires a

declaration

), or if you

wish to

define an

external

SYSTEM

DTD in the

DOCTYPE

but use

standalone

=yes

(redundant

?), you

must use

an XML

Declaratio

n for

XHTML,

but this
http://wiki.whatwg.org/wiki/FAQ#How_do_I_specify_the_character_encoding.3Fhttp://wiki.whatwg.org/wiki/FAQ#How_do_I_specify_the_character_encoding.3Fhttp://wiki.whatwg.org/wiki/FAQ#How_do_I_specify_the_character_encoding.3Fhttp://wiki.whatwg.org/wiki/FAQ#How_do_I_specify_the_character_encoding.3Fhttp://wiki.whatwg.org/wiki/FAQ#How_do_I_specify_the_character_encoding.3F


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may not be

allowable

in the

future in

HTML.

For futurecompatibili

ty, it would

be best to

avoid

XML 1.1-

only

markup,

convert to

UTF-8 or

UTF-16

(probablyUTF-8

which

could allow

use of a

meta tag),

and avoid

use of a

SYSTEM

DTD

(rendering

thestandalone

=yes

unnecessar

y),

respectivel

y. Do not

use ameta

tag, unless

it is UTF-8

(and

included inthe first

512 bytes

of the

document),

in which

case it is

probably a


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good idea

to include

it for the

sake of

HTML (as

)

in case you

cannot

specify

such in a

content

header.

Namesp

aced

elements

Elements and attributes for

known vocabularies (HTML,

SVG and MathML) areimplicitly assigned to

appropriate namespaces,

according to the rulesspecified in the parsing

algorithm. Elements in the

HTML, SVG, or MathML

namespaces may have anxmlns attribute explicitly

specified, if, and only if, it

has the exact value

"http://www.w3.org/1999/xhtml" (see namespace

declaration). The attribute has

absolutely no effect. It is

basically a talisman. It isallowed merely to make

migration to and from

XHTML mildly easier. Whenparsed by an HTML parser,

the xmlns attribute itself ends

up in no namespace. Foreign

elements are also not treatedas being in another

namespace and will have no

effect except for displayingby default as inline elements

(and be aware that self-

closing elements cannot beused as such since

The XHTML namespace

must be declared for HTML

elements according to therules defined by the

Namespaces in XML

specification. Namespacesmust be explicitly declared.

The xmlns attribute ends up

in the"http://www.w3.org/2000/

xmlns" namespace. Foreign

elements can be used

independently of HTML

elements, as long as they areassigned to their own

namespace.

Declare

HTML

namespaces(or other

namespaces

) explicitlyand do not

prefix

XHTML

elements.

Do not

depend on

the

behavior offoreign

namespace

d elements

in an

HTML

setting; if

you need to

include

these, you

will

probablywish to set

this foreign

markup

via CSS todisplay:n

one. You

should
http://www.w3.org/1999/xhtmlhttp://www.w3.org/1999/xhtmlhttp://wiki.whatwg.org/wiki/FAQ#What_is_the_namespace_declaration.3Fhttp://wiki.whatwg.org/wiki/FAQ#What_is_the_namespace_declaration.3Fhttp://www.w3.org/TR/REC-xml-names/http://www.w3.org/2000/xmlnshttp://www.w3.org/2000/xmlnshttp://www.w3.org/1999/xhtmlhttp://www.w3.org/1999/xhtmlhttp://wiki.whatwg.org/wiki/FAQ#What_is_the_namespace_declaration.3Fhttp://wiki.whatwg.org/wiki/FAQ#What_is_the_namespace_declaration.3Fhttp://www.w3.org/TR/REC-xml-names/http://www.w3.org/2000/xmlnshttp://www.w3.org/2000/xmlns


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unrecognized elements will

be treated as though they are

non-void; thus one cannot, forexample, type

in HTML or it will be treatedas though there is no

immediate closing tag).

Namespaced prefixes are notallowed on HTML elements;

a prefixed xmlns attribute

cannot be used even if it isdefined in the XHTML

namespace.

explicitly

close (not

self-close)

all empty

elements

defined ina non-

XHTML

namespace

, since

otherwise

when used

in HTML,

HTML will

treat them

as though

they havenot been

closed.

Namesp

aced

attribut

es on

HTMLelement

s

Attributes of the form

xmlns:prefix may not be used

on HTML elements.

The xmlns:prefix attributes

end up in the"http://www.w3.org/2000/

xmlns" namespace.

Do not use

namespace

d

attributes

on HTML

elements.

Do not

depend on

thebehavior of

foreign

attributes

in an

HTML

setting.

Namesp

ace

attribut

es on

foreign

element

s

Elements in the SVG

namespace may have anxmlns attribute specified, if,

and only if, it has the exact

value"http://www.w3.org/2000/

svg". The attribute is optional

because the namespace isimplied during parsing.

Elements in the MathML

The SVG and MathML

namespaces must be declared

for SVG and MathMLelements, respectively,

according to the rules definedbyNamespaces in XML. Thexmlns and xmlns:prefix

attributes end up in the"http://www.w3.org/2000/

xmlns" namespace.
http://www.w3.org/2000/xmlnshttp://www.w3.org/2000/xmlnshttp://www.w3.org/2000/svghttp://www.w3.org/2000/svghttp://www.w3.org/2000/xmlnshttp://www.w3.org/2000/xmlnshttp://www.w3.org/2000/xmlnshttp://www.w3.org/2000/xmlnshttp://www.w3.org/2000/svghttp://www.w3.org/2000/svghttp://www.w3.org/2000/xmlnshttp://www.w3.org/2000/xmlns


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namespace may have an

xmlns attribute specified, if,

and only if, it has the exactvalue

"http://www.w3.org/1998/Math/MathML". The attribute

is optional because the

namespace is implied duringparsing.

Foreign elements may also

have an xmlns:xlink

attribute specified, if, andonly if, it has the exact value"http://www.w3.org/1999/

xlink". This attribute isoptional, even if XLinkattributes are used, because

the namespaces for XLink

attributes is implied during

parsing.

When parsed by an HTML

parser, the xmlns and

xmlns:xlink attributes end

up in the"http://www.w3.org/2000/

xmlns" namespace.

XLink

attribut

es

Foreign elements may use the

attributes xlink:actuate,

xlink:arcrole,

xlink:href, xlink:role,

xlink:show, xlink:title

and xlink:type. These

attributes are placed in the"http://www.w3.org/1999/

xlink". The prefix used must

be "xlink".

XLink attributes may be

specified on foreign elements

using any prefix, subject tothe conformance rules defined

byNamespaces in XML. The

XLink namespace must bedeclared according to the

conformance rules defined byNamespaces in XML if XLink

attributes are used within thedocument.

Do not use

XLink

attributes

on HTML

elements

and do not

depend on

them on

foreign

elements aswill not

work as

such in

HTML. If

being used,

ensure theyhave the
http://www.w3.org/1998/Math/MathMLhttp://www.w3.org/1998/Math/MathMLhttp://www.w3.org/1999/xlinkhttp://www.w3.org/1999/xlinkhttp://www.w3.org/2000/xmlnshttp://www.w3.org/2000/xmlnshttp://www.w3.org/1999/xlinkhttp://www.w3.org/1999/xlinkhttp://www.w3.org/1998/Math/MathMLhttp://www.w3.org/1998/Math/MathMLhttp://www.w3.org/1999/xlinkhttp://www.w3.org/1999/xlinkhttp://www.w3.org/2000/xmlnshttp://www.w3.org/2000/xmlnshttp://www.w3.org/1999/xlinkhttp://www.w3.org/1999/xlink


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appropriate

XLink

namespacedefined.

XML

attribut

es

Foreign elements may use theattributes xml:lang, xml:id,

xml:base and xml:space.

These attributes are placed in

the"http://www.w3.org/XML/1

998/namespace". The prefix

used must be "xml".

HTML elements may use thexml:lang attribute. The

attribute in no namespacewith no prefix and with the

literal localname "xml:lang"

has no effect on languageprocessing (as does "lang".

HTML elements must not use

the xml:base, xml:space, or

xml:id attributes.

Any element, includingHTML elements, may use the

attributes xml:lang, xml:id,

xml:base and xml:space.

These attributes are placed in

the"http://www.w3.org/XML/1

998/namespace". The prefix

used must be "xml".

Though

they can beused on

foreign

elements,

do not use

xml:base,

xml:id, orxml:space

on HTML

elements;

use both

xml:langand lang

attributes

whenever

one is to be

needed on

HTML

elements.

Attribut

es

Names are not case sensitive.

Attribute minimization isallowed (i.e. omitting the

equals sign and the value).

Names are case sensitive (and

lower case). Attributeminimization is not allowed.

Use lower

caseattribute

names. Donotminimize

attributes.

Non-namespaced

attributes

not

belongingto HTML

will be

included inthe DOM

tree and

accessibleto script

and

stylesheets,
http://www.w3.org/XML/1998/namespacehttp://www.w3.org/XML/1998/namespacehttp://www.w3.org/XML/1998/namespacehttp://www.w3.org/XML/1998/namespacehttp://www.w3.org/XML/1998/namespacehttp://www.w3.org/XML/1998/namespacehttp://www.w3.org/XML/1998/namespacehttp://www.w3.org/XML/1998/namespace


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but it is

discouraged

to use thesedue to the

potential

for futurenaming

conflicts;data-

attributes

can be used

instead, orif in an

XML-only

environment,

namespacedattributes.

Attribut

e values

White space characters arenot normalized. Unquoted

attribute values are allowed.

Fixed or default attributevalues ...?

White space characters arenormalized to single spaces

(unless attribute is of CDATA

type?). Unquoted attributevalues are not allowed.

Default attribute values could

conceivably be defined with aDTD.

Createwhitespace

in attribute

valueswhich is

already

normalized(converted

to single

spaces).Alwaysquote

attribute

values. Do

not rely on

defining

default or

fixed

attribute

values (or

elementswith

exclusively

element

content) in

a DTD

(unless it

matches
http://www.w3.org/TR/REC-xml/#AVNormalizehttp://www.w3.org/TR/REC-xml/#AVNormalize


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HTML

behavior).

Space

charact

ers

The space characters are

defined as:

U+0009CHARACTER

TABULATION

U+000A LINE FEED

U+000C FORM

FEED

U+000D CARRIAGERETURN

U+0020 SPACE

The space characters aredefined as:

U+0009

CHARACTER

TABULATION

U+000A LINE FEED

U+000D CARRIAGE

RETURN

U+0020 SPACE

The

difference is

theinclusion of

Form

Feed.Form

feed

charact

ers arediscour

aged in

XML1.1.

Do not use

the formfeed

character.

The

DOCTY

PE

A DOCTYPE is a mostly

useless, but required, header.

The DOCTYPE is usedduring parsing to determing

the parsing mode. The

keywords "DOCTYPE",

"PUBLIC" and "SYSTEM", and

the name "html" are treated

case insensitively. The systemidentifier"about:legacy-

compat" (and the public and

system identifiers forprevious versions of HTML)

are case sensitive.

Conforming HTML

documents are required to use (case

insensitively) or the legacy-

compat version .

When using the obsolete but

conforming DOCTYPEs

based on the HTML 4.0 and4.01 Strict DTDs, the system

The DOCTYPE is optional.

XML rules for case sensitivity

apply (everything is casesensitive).

Either of the DOCTYPEs

defined in HTML5 may be

used, or any other customDOCTYPE. If the public

identifier is specified, the

system identifier must also bespecified. The obsolete status

of the obsolete permittedDOCTYPEs defined forHTML does not apply to

XHTML. Any DOCTYPE

may be used, subject to the

conformance rules defined byXML.

Use of an internal subset is

permitted according to therequirements of XML. Some

validating XML processors

may dereference the system

identifier, if used, but mostbrowsers use non-validating

Use the

empty

DOCTYPE

with no

SYSTEM

or

PUBLIC

identifiers

and no use

of internet

subset.


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identifier is optional. The

obsolete but conforming

DOCTYPEs based on

XHTML 1.0 Strict and

XHTML 1.1 may also bespecified.

Use of an internal subset isforbidden. The system

identifier is never de-

referenced by HTML

implementations.

processors.

Element

names

Element names are case

insensitive.

Element names are case

sensitive and lower-case.

Only use

lower-case

element

names (aswith

attributes).

Void vs.

Non-

void

Element

s

Void elements only have a

start tag; end tags must not bespecified for void elements,

and it is impossible for them

to contain any content. Atrailing slash may optionally

be inserted at the end of the

element's tag, immediatelybefore the closing greater-

than sign. For non-void

elements (e.g., ), the

trailing slash is a parsing error(ignored and thus treated as

unclosed).

Void elements may use either

the empty-element tag syntax(EmptyElemTag) or use a start

tag immediately followed by

an end tag, with no content inbetween. While it is possible

for the element to contain

content, this is non-conforming.

For void

elements

(e.g.,

), do

not include

content or

use a

closing tag;

only use a

self-closing

element

with

closing

slash at the

end (with a

space

preceding

it for the

sake of

older

browsers).

For non-

void

elements,

i.e., where

content


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can exist

(e.g.,

),

always use

an explicit

closing tag(not a self-

closing tag)

even if

there is no

content.

Unexpe

cted end

tags

Unexpected end tags (in

HTML, an unexpected

or
can cause the start tagto be implied before it).
Unexpected end tags are well-

formedness errors.

Do not add

end tags

unless thereis an

explicit and

properlynested open

tag before

it.

End tag

with

attribut

es

?An end tag with attributes is

not allowed.

Do not useend tags

with

attributes.

Raw

text

elements

RCDAT

A

element

s

Foreign

element

s

Normal

element

s

Optiona

l tags

Forsome elements, the start

and/or end tags are optional

and are implied by certainspecified conditions. For

example, the end tag for the p

element is implied by a

End tags must be explicitly

included for all elements,

except empty elements usingtheEmptyElemTagsyntax.

Always use

end tags (or

self-closingtags for

void

elements).
http://wiki.whatwg.org/wiki/HTML_vs._XHTML#HTML_Elements_with_Optional_Tagshttp://wiki.whatwg.org/wiki/HTML_vs._XHTML#HTML_Elements_with_Optional_Tagshttp://wiki.whatwg.org/wiki/HTML_vs._XHTML#HTML_Elements_with_Optional_Tags


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subsequent p element.

Omitting the end tag for other

elements is a parse error andvarious error recovery

procedures are appliedappropriately.

Comme

nt

syntax

Comments must start with thefour character sequence

"" (bogus comments such

as those beginning with "


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R-THAN

SIGN ('>')

character.

Processing

Instruct

ions

HTML does not allow

processing instructions anddeprecates the boguscomments which appear in

their form, whether in the

form .

XHTML allows the use ofXML processing instructions

which are only closed by "?

>".

Avoid ">"

inside

processinginstruction

s (as these

will close

the

"instructio

n"

(comment)

prematurel

y) (or one

must strip

outprocessing

instruction

s entirely).

Processing

instruction

s might

need to be

avoided

entirely in

case

HTMLmay in

future

disallow

them

completely.

CDATA

sections

is a a

bogus comment. The

sequence of characters "]]>"

in content when it does notmark the end of a CDATA

section is just regular

character data.

is a CDATA

section. The sequence of

characters "]]>" in content

when it does not mark the endof a CDATA section is a well-

formedness error.

Ensuresequence

"]]>" in

content isescaped

(not

necessaryto escape in

attribute

values). Do

not use

CDATA


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sections.

Unescap

ed

Special

Charact

ers

Unescaped ampersands

(U+0026 AMPERSAND - &,

instead of&) are

permitted within the content

ofnormal elements,RCDATAelements,foreign elements

and attribute values where

they are not considered to beambiguous ampersands, andwithinRaw text elements.

Unescaped less than signs

(U+003C LESS-THAN SIGN

-


16/18

er data

valid

set of

unicodecharact

ers in

XML1.0 is

limited

beyond

that inHTML

(we

need tospecify

this

here).

Element-specific parsing

In HTML, the script and style elements are parsed as CDATA elements. (Note:

the definition ofCDATA differs from that in XML). In XML, they're parsed as

normal elements (which means that things that look like comments are treated as

realcomments, and things that look like start tags actually are start tags).

In HTML, the title and textarea elements are parsed as RCDATA elements.

(Note: The definition ofRCDATA differs from that in SGML and there is no

RCDATA in XML).

In HTML, if scripting is enabled, the noscript element is parsed as an CDATA

element. If scripting is disabled, it's parsed as a normal element. In XHTML, theelement is always parsed as a normal element, and can't really be used to stop

content from being present when script is disabled.

In HTML, the iframe, noembed and noframes elements are parsed as CDATA

elements. In XHTML, they are parsed as normal elements, and therefore do not

stop content from being used.

In HTML, tags for certain elements, which appear out of context, are ignored.

This includes caption, col, colgroup, frame, frameset, head, option,

optgroup, tbody, td, tfoot, th, thead, tr.

In XHTML, table elements may contain child tr elements. In the HTML

serialisation, due to backwards compatibility constraints, this is not possible

(though it may be done through DOM manipulation). The plaintext element has a special parsing requirement in HTML. (It is,

however, forbidden.)

In HTML, a line feed that immediately follows a pre, listing ortextarea start

tag is ignored.

Many other special handling of edge cases and error conditions, not all of whichare listed here, occur in HTML. (such as?)


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The following are void elements in HTML (see void elements in table): In head

(base, link, meta), in body (area,br, col, embed, hr, img, input, param)

HTML Elements with Optional Tags

Element Start Tag End Tag

html optional optional

head optional optional

body optional optional

li required optional

dt required optional

dt required optional

p required optional

colgroup optional optional

thead required optionaltbody optional optional

tfoot required optional

tr required optional

th required optional

td required optional

rt required optional

rp required optional

optgroup required optional

option required optional

Scripts

document.write() and document.writeln() cannot be used in XHTML, they

can in HTML.

In XHTML, the use of the innerHTML property requires that the string be a well-

formed fragment of XML.

DOM APIs are case sensitive in XHTML and some are case insensitive in HTML.(This does not apply to elements which are not in the HTML namespace)

o Element.tagName and Node.nodeName return the value in uppercase inHTML but lower-case in XHTML (Node.localName is consistent now, asof HTML5).

o Document.createElement() is case insensitive (the canonical form is

lowercase).o Element.setAttributeNode() will change the attribute name to lowercase.

o Element.setAttribute() is case insensitive (the canonical form is

lowercase).


18/18

o Document.getElementsByTagName() and

Element.getElementsByTagName() are case insensitive.o Document.renameNode(). If the new namespace is the HTML namespace,

then the new qualified name will be lowercased before the rename takes

place.

In HTML, Document.createElement() will create an element in the HTMLnamespace. In XML (including XHTML), the namespace is defined by both

DOM2 and DOM3 to be null.o In XHTML, browsers lack interoperability in this area. In Firefox and

Safari, the namespace is dependent upon the MIME type. In Opera, it's

dependent upon the root element.

XPath expressions targeted at pre-HTML5 browsers need to use the XHTMLnamespace for XHTML and null for HTML. (HTML5 browsers would use the

XHTML namespace even in HTML.)

Stylesheets

Selectors, as used in CSS, match case sensitively in XHTML, but caseinsensitively in HTML.

CSS requires special handling of the body element in HTML for painting

backgrounds on the canvas, which do not apply to XHTML.

For polyglot documents, use lower-case element selectors and style the html and

body elements appropriately (?).

differences between html and xhtml

Documents