HTML vs. XHTML

Differences Between HTML and XHTML

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.
Differences marked @@@ are differences that could theoretically be changed without affecting 
backwards compatibility.

MIME Types

• XHTML must be served with an XML MIME type, such as application/xml or application/xhtml+xml.
• HTML must be served as text/html.

It is the MIME type that determines what type of document you are using. If you use attempt to send XHTML as text/html, you are actually just using HTML, possibly with syntax errors.

Technically, according to the spec, XHTML 1.0 is allowed to be served as text/html. But, due to the above reason, such a document is considered to be an HTML document, not an XHTML document.

Parsing

XHTML uses XML parsing requirements. HTML uses its own which are defined much more closely to the way browsers actually handle HTML today.

• In XHTML, well-formedness errors are fatal. In HTML, error handling rules are much more graceful. Well-formedness errors, which are also syntax errors in HTML, include the following:
  • Unencoded ampersands (& instead of &), and less than signs (< instead of <) (This does not apply to CDATA).
  • Comments containing extra pairs of hyphens or ending with a hyphen. e.g.
    • <!-- syntax -- error --> or
    • <!-- syntax error --->.
  • Mismatched end tags (does not apply to elements with optional tags)
  • Unclosed tags.
  • Unexpected characters occuring in or before attribute names.
  • Unexpected occurrence of EOF.
  • Unexpected characters before the DOCTYPE name.
  • Missing DOCTYPE name.
  • A PUBLIC identifer in a DOCTYPE without a SYSTEM identifier (Note: including either of these is a syntax error in HTML5; but, in XML only the SYSTEM identifier is allowed to occur on its own).
  • End tags with attributes.
  • Unexpected end tags (in HTML, an unexpected </br> or </p> can cause the start tag to be implied before it).
• The internal subset is permitted in XML, but meaningless (and forbidden) in HTML.
  • In some cases, an internal subset in HTML would end up being partly rendered inline.
• The sequence of characters "]]>" when it does not mark the end of a CDATA section is a well-formedness error in XHTML, but valid in HTML.
• In XHTML: <![CDATA[...]]> is a CDATA section. In HTML, it's a bogus comment.
• In XHTML, <?foo ...?> is a processing instruction. In HTML, it's a bogus comment.
• In HTML, the trailing slash used for the empty element syntax is a parse error for non-void elements (see below), but is ignored in all cases.
• In HTML, the script and style elements are parsed as CDATA. (Note: the definition of CDATA differs from that in XML). In XML, they're parsed as normal elements (which means that comments are treated as real comments, and things that look like start tags actually are start tags).
• In HTML, the title and textarea elements are parsed as RCDATA. (Note: The definition of RCDATA differs from that in SGML and there is no RCDATA in XML).
• In HTML, if scripting is enabled, the noscript element is parsed as CDATA. If scripting is disabled, it's parsed as PCDATA. In XHTML, the element has no effect, 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. In XHTML, they are parsed as normal elements, and therefore do not stop content from being used.
• White space characters in attribute values are normalized to spaces in XHTML.
• Elements with optional tags are implied in certain conditions.
• In HTML, base, link, meta, style and title elements with tags occurring in the body are moved inserted into the head. In XHTML, they stay where they were specified.
• 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.
• The plaintext element has a special parsing requirement in HTML. (it is, however, forbidden).
• Many other special handling of edge cases and error conditions, not all of which are listed here, occur in HTML.

Syntax

• In HTML, the doctype is required. In XHTML, it is optional.
• In XHTML, tag names and attribute names are case sensitive. In HTML, they are case insensitive.
• In XHTML, non-empty elements require both a start and an end tag. In HTML, certain elements allow the omission of either or both:
  • html (both)
  • head (both)
  • body (both)
  • li (end tag)
  • dt (end tag)
  • dd (end tag)
  • p (end tag)
  • colgroup (both)
  • thead (end tag)
  • tbody (both)
  • tfoot (end tag)
  • tr (end tag)
  • td (end tag)
  • th (end tag)
• In XHTML, empty elements may use either the empty element syntax (<br/>) or have an end tag immediately follow the start tag (<br></br>). In HTML, the empty element syntax (trailing slash) is allowed on void elements, but forbidden on other elements. However, it serves no purpose whatsoever and can be omitted. End tags for void elements are forbidden.
  • base, link, meta, hr, br, img, embed, param, area, col and input
  • Note: the following are treated as void elements for the purpose in the parsing requirements, but, as they are obsolete and non-standard, the trailing slash is not permitted: basefont, bgsound, spacer, wbr. (although, since these elements are not permitted anyway, it doesn't make much difference).
• HTML allows attribute minimisation (i.e. omitting the value), XHTML does not.
• HTML allows the use of unquoted attribute values, XHTML does not.
• XHTML allows the use of CDATA sections, HTML does not.
• XHTML allows the use of processing instructions, HTML does not.
• In HTML, all entity references are predefined and do not require a DTD. But because there is no DTD for XHTML5, entity references cannot be used in XHTML. (excluding the 5 predefined entities: &, <, >, " and ')
  • You may provide your own DTD for use with your own validating parser, but be aware that browsers do not use validating parsers and will not read the DTD.
• The valid set of unicode characters in XML 1.0 is limited beyond that in HTML.
• Namespace prefixes are permitted in XHTML. They are forbidden in HTML.

Markup

• The namespace declaration (xmlns attribute) is required in XHTML. The xmlns attribute is also allowed to appear on the html element in HTML on the condition that is has the value "http://www.w3.org/1999/xhtml".
  • <html xmlns="http://www.w3.org/1999/xhtml">
  • In HTML, the xmlns attribute has absolutely no effect. It is basically a talisman. It is allowed merely to make migration to and from XHTML mildly easier. When parsed by an HTML parser, the attribute ends up in the null namespace
  • In XML (with an XML Namespaces-aware parser), an xmlns attribute is part of the namespace declaration mechanism, and an element cannot actually have an xmlns attribute in the null namespace. In DOM implementations, the attribute ends up in the "http://www.w3.org/2000/xmlns/" namespace.
• XHTML allows non XHTML elements and attributes (in different namespaces) to be used, HTML does not.
• XHTML uses the xml:lang attribute, HTML uses lang instead,
• XML ID introduces xml:id, which could be used in XHTML. In HTML it has no effect.
• In HTML, the noscript element may be used. In XHTML, it is forbidden.
• HTML uses the base element, XHTML uses xml:base instead.
• In XHTML, p elements may contain structured inline level elements including blockquote, dl, menu, ol, ul, pre and table. In the HTML serialisation, due to backwards compatibility constraints, this is not possible (though it may be done through DOM manipulation).
• 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).

Character Encoding

• In XHTML, the XML declaration may be used to specify the character encoding. In HTML, the xml declaration is forbidden
• In HTML, the meta element may be used insted. The http-equiv attribute on the meta element is forbidden in XHTML and is ignored if included.
• The default character encoding for XHTML is, according to XML rules, UTF-8 or UTF-16. If the encoding is unspecified in HTML, it should be determined through implementation specific heuristics or fallback to a default value (Note: this section of the spec is not yet finished).

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)
  • Element.tagName, Node.nodeName, and Node.localName return the value in uppercase.
  • Document.createElement() is case insensitive (the canonical form is lowercase).
  • Element.setAttributeNode() will change the attribute name to lowercase.
  • Element.setAttribute() is case insensitive (the canonical form is lowercase).
  • Document.getElementsByTagName() and Element.getElementsByTagName() are case insensitive.
  • Document.renameNode(). If the new namespace is the HTML namespace, then the new qualified name must be lowercased before the rename takes place.
• In HTML, Document.createElement() will create an element in the HTML namespace. In XML (including XHTML), the namespace is defined by both DOM2 and DOM3 to be null.
  • In XHTML, browsers lack interoperability in this area. In Firefox, the namespace is dependent upon the MIME type. In Opera, it's dependent upon the root element and in Safari, it's always null.

Stylesheets

• Selectors, as used in CSS, match case sensitively in XHTML, but case insensitively in HTML.
• CSS requires special handling of the body element in HTML for painting backgrounds on the canvas, which do not apply to XHTML.

Differences Between HTML 4.01 and HTML 5

MIME Type

Both HTML 4.01 and HTML 5 use text/html.

Content-Type: text/html; charset=UTF-8

Parsing HTML

• HTML 2.0 to HTML 4.01 were formally based on SGML, but browsers did not implement SGML parsers. 4.2 SGML and B.3 SGML implementation notes, HTML 4.01. This is a non-normative section of HTML 4.01 specification. And it already makes the difference between HTML user agents and SGML user agents.
• HTML 5 is defines its own parsing requirements based on the way browsers actually handle HTML.

Syntax

• TODO

Markup

• List of HTML 4.01 elements
• List of HTML 4.01 attributes
• HTML5 Elements and Attributes

Obsolete Attributes

Some attributes that were defined in HTML4 are not included in HTML5. Here's a current list (subject to change, see the spec):

• html@version
• head@profile
• a@rev, link@rev
• a@target, area@target, base@target, form@target (is mentioned in WF2...), link@target
• a@charset, link@charset, script@charset
• table@summary
• td@headers, th@headers
• td@axis, th@axis
• param@valuetype
• object@standby
• meta@scheme
• object@archive

In addition, HTML5 has none of the presentational attributes that were in HTML4 (including those on <table>). Any attributes defined on elements that are not in HTML5 are (obviously) also not in HTML5.

Obsolete Elements

The following elements were present in HTML4 but are not defined in HTML5:

• acronym (use instead)
• applet (use <object> instead)
• basefont
• big
• center
• dir
• font
• frame
• frameset
• isindex
• noframes
• noscript (only in XHTML)
• s
• strike
• tt
• u

Character Encoding

HTML 4 Algorithm

Source 5.2.2 Specifying the character encoding, HTML 4.01 Specification.

1. An HTTP "charset" parameter in a "Content-Type" field.
2. A META declaration with "http-equiv" set to "Content-Type" and a value set for "charset".
3. The charset attribute set on an element that designates an external resource.

HTML 5 Algorithm

The exact algorithm that browsers must follow in order to determine the character encoding is specified in HTML 5. The basic algorithm works as follows:

1. If the transport layer specifies an encoding, use that, and abort these steps. (e.g. The HTTP Content-Type header).
2. Read the first 512 bytes of the file, or at least as much as possible if less than that.
3. If the file starts with a UTF-8, UTF-16 or UTF-32 BOM, then use that and abort these steps.
4. Otherwise use the special algorithm to search the first 512 bytes for a meta element that declares the encoding. The algorithm is relatively lenient in what it will detect, though since it doesn't use the normal parsing algorithm, there are some restrictions.

Differences Between DOM Level 2.0, 3.0 and the HTML 5 DOM APIs

This section might belong on a separate page.

• TODO (need to talk about the changes to the DOM API that HTML5 is making, compared with DOM2 and DOM3)

Translations

• German translation: In progress (Jens Meiert)