XML 宣言の省略と XHTML Media Types の Second Edition についてのまとめ

Be aware that processing instructions are rendered on some user agents. However, also note 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.

DO NOT include XML processing instructions NOR the XML declaration.

Rationale: Some HTML 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 HTML browsers, you should avoid using processing instructions and XML declarations.

Consequence: 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. See, however, guideline 9 below.

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.

If an element has an EMPTY content model DO use the minimized tag syntax permitted by XML (e.g., <br />). DO NOT use the alternative syntax (e.g., <br></br>) allowed by XML, since this may be unsupported by HTML user agents. Also, DO include a space before the trailing / and >.

Empty elements in the XHTML family include: area, base, basefont, br, col, hr, img, input, isindex, link, meta, and param.

Rationale: HTML user agents ignore the  /> at the end of a tag, but without it they may incorrectly parse the tag or its attributes. HTML user agents also may not recognize the alternate syntax permitted by XML.

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 />).

If an element permits content (e.g., the p element) but an instance of that element has no content (e.g., an empty paragraph), DO NOT use the "minimized" tag syntax (e.g., <p />).

Rationale: HTML user agents may give uncertain results when using the the minimized syntax permitted by XML when an element has no content.

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 implementations.

DO use external style sheets if your style sheet uses < or & or ]]> or --. DO NOT use an internal stylesheet if the style rules contain any of the above characters.

DO use external scripts if your script uses < or & or ]]> or --. DO NOT embed a script in a document if it contains any of these characters.

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.

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Avoid line breaks and multiple whitespace characters within attribute values. These are handled inconsistently by user agents.

DO ensure that attribute values are on a single line and only use single whitespace characters. DO NOT use line breaks and multiple consecutive white space characters within attribute values.

Rationale: These are handled inconsistently by user agents.

Don't include more than one isindex element in the document head. The isindex element is deprecated in favor of the input element.

Use both the lang and xml:lang attributes when specifying the language of an element. The value of the xml:lang attribute takes precedence.

DO use both lang and xml:lang attributes when specifying the language of an element in markup languages that support the use of both.

DO NOT use the only the lang attribute, even in languages that include it such as XHTML 1.0.

Rationale: HTML 4 documents use the lang attribute to identify the language of an element. XML documents use the xml:lang attribute. CSS has a "lang" pseudo selector that automatically uses the appropriate attribute depending on the document type. Therefore, specifying both attributes ensures that single CSS selectors will work in both modes.

In XML, URIs [RFC2396] that end with fragment identifiers of the form "#foo" do not refer to elements with an attribute name="foo"; rather, they refer to elements with an attribute defined to be of type ID, e.g., the id attribute in HTML 4. Many existing HTML clients don't support the use of ID-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 type CDATA, the type of the name attribute has been changed to NMTOKEN. This attribute is constrained such that it can only have the same values as type ID, or as the Name production in XML 1.0 Section 2.5, 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.

Finally, note that XHTML 1.0 has deprecated the name attribute of the a, applet, form, frame, iframe, img, and map elements, and it will be removed from XHTML in subsequent versions.

DO use the id attribute to identify elements.

DO ensure that the values used for the id attribute are limited to the pattern [A-Za-z][A-Za-z0-9:_.-]*.

DO NOT use the name attribute to identify elements, even in languages that permit the use of name such as XHTML 1.0.

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 attribute is defined to be of type ID. Relying upon the id attribute as an anchor will work well in modern HTML and XHTML-aware user agents.

To specify a character encoding in the document, use both the encoding attribute specification on 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"' />). The value of the encoding attribute of the xml processing instruction takes precedence.

DO encode your document in UTF-8 or UTF-16. When delivering the document from a server, DO set the character encoding for a document via the charset parameter of the HTTP Content-Type header. When not delivering the document from a server, DO set the encoding via a "meta http-equiv" statement in the document (e.g., <meta http-equiv="Content-Type" content="text/html; charset=EUC-JP" />). However, note that doing so will explicitly bind the document to an a single content type.

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 HTML and XML user agents. When that mechanism is not available, the only portable fallback is the "meta http-equiv" statement.

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.

DO use the full form for boolean attributes, as required by XML (e.g., disabled="disabled"). Such attributes include: compact, nowrap, ismap, declare, noshade, checked, disabled, readonly, multiple, selected, noresize, and defer.

Rationale: The compact form of these attributes is not well formed XML, and therefore invalid.

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:

1. Applications 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.
2. Applications that access XHTML documents served as Internet media types text/xml or application/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. the tbody element within table). 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. Applications need to adapt to this accordingly.

DO rely upon the HTML 4 DOM as defined in The Document Object Model level 1 Recommendation [DOM] for scripting. This means, in particular, that the names of elements and attributes will be returned (from functions that return such things) in upper case.

Rationale: Using the HTML DOM will result in maximum portability of scripts, since the HTML DOM is supported in both HTML and XHTML documents in modern user agents.

When an attribute value contains an ampersand, it must be expressed as a character entity reference (e.g. "&"). For example, when the href attribute of the a element refers to a CGI script that takes parameters, it must be expressed as http://my.site.dom/cgi-bin/myscript.pl?class=guest&name=user rather than as http://my.site.dom/cgi-bin/myscript.pl?class=guest&name=user.

DO ensure that when content or attribute values contain the reserved character & it is used 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.

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The Cascading Style Sheets level 2 Recommendation [CSS2] defines style properties which are applied to the parse tree of the HTML or XML document. 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:

1. CSS style sheets for XHTML should use lower case element and attribute names.
2. 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.
3. Within the XHTML name space, 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.
4. Within the XHTML name space, user agents are expected to recognize the "class" attribute. Therefore, style sheets should be able to continue using the shorthand "." selector syntax.
5. 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 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).

Rationale: These simple rules will help increase the portability of CSS rules regardless of the media type the document is processed as.

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DO NOT use xml stylesheet declarations to identify style sheets.

DO use the style or link elements to define stylesheets.

Rationale: Since XML processing instructions may be rendered by some HTML 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 interpret stylesheets referenced from those elements, documents constructed to use them will work as expected.

Rationale: This character is recognized as white space in HTML 4, but is NOT considered white space in XML.

DO use ' to specify an escaped apostrophe. DO NOT use '.

Rationale: The entity ' is not defined in HTML 4.