HTML 4.01 validity errors

This is a collection of notes collected as research for a proposal for WCAG 2.0 Guideline 4.1 Level 1 Success Criterion. It has not been reviewed or approved by the WCAG WG. For questions or comments please contact Wendy Chisholm.

Common HTML 4.01 validity errors and their effect on accessibility

• Some of these errors copied from the report, "A comparative assessment of Web accessibility and technical standards conformance" others

  from "Explanation of the error messages for the W3C Markup Validator".

• DTD references are to "HTML Transitional."

misc. notes and references

HTML 4.01 section 19.1 Document Validation:

Beware that such validation, although useful and highly recommended, does not guarantee that a document fully conforms to the HTML 4 specification. This is because an SGML parser relies solely on the given SGML DTD which does not express all aspects of a valid HTML 4 document. Specifically, an SGML parser ensures that the syntax, the structure, the list of elements, and their attributes are valid. But for instance, it cannot catch errors such as setting the width attribute of an IMG element to an invalid value (i.e., "foo" or "12.5"). Although the specification restricts the value for this attribute to an "integer representing a length in pixels," the DTD only defines it to be CDATA, which actually allows any value. Only a specialized program could capture the complete specification of HTML 4.

• 3 On SGML and HTML
• does SGML discuss proper nesting? well-formedness? There is a note in section 3.2.1..."Please consult the SGML standard for information about rules governing elements (e.g., they must be properly nested, an end tag closes, back to the matching start tag, all unclosed intervening start tags with omitted end tags (section 7.5.1), etc.)."
• from XHTML spec 4.1, "Although overlapping is illegal in SGML, it is widely tolerated in existing browsers." OK - so although "well-formedness" is an XML thing, "overlapping" and "nesting" are concepts from SGML that should apply to HTML.
• also from XHTML spec 4.9, "SGML gives the writer of a DTD the ability to exclude specific elements from being contained within an element. Such prohibitions (called "exclusions") are not possible in XML.

  For example, the HTML 4 Strict DTD forbids the nesting of an 'a' element within another 'a' element to any descendant depth. It is not possible to spell out such prohibitions in XML. Even though these prohibitions cannot be defined in the DTD, certain elements should not be nested. A summary of such elements and the elements that should not be nested in them is found in the normative Element Prohibitions."

  cool! so, what are they?

• xhtml element prohibitions only a few elements. rest in the DTD? HTML 4.01 DTD specifies that A can not be nested within A, "<!ELEMENT A - - (%inline;)* -(A) " another type of exclusion from the DTD - <!ENTITY % pre.exclusion "IMG|OBJECT|BIG|SMALL|SUB|SUP"> <!ELEMENT PRE - - (%inline;)* -(%pre.exclusion;) -- preformatted text -->
• don't see that tables are defined in the DTD to allow nesting...

Notes from "A Gentle Introduction to SGML"

From these rules, it may be inferred that we do not need to mark the ends of stanzas or lines explicitly. From rule 2 it follows that we do not need to mark the end of the title---it is implied by the start of the first stanza. Similarly, from rules 3 and 1 it follows that we need not mark the end of the poem: since poems cannot occur within poems but must occur within anthologies, the end of a poem is implied by the start of the next poem, or by the end of the anthology. Applying these simplifications, we could mark up the same poem as follows:...

from 4 SGML Structures. End elements do not need to be provided.

The second part of the declaration specifies what are called minimization rules for the element concerned. These rules determine whether or not start- and end-tags must be present in every occurrence of the element concerned. They take the form of a pair of characters, separated by white space, the first of which relates to the start-tag, and the second to the end-tag. In either case, either a hyphen or a letter O (for ``omissible'' or ``optional'') must be given; the hyphen indicating that the tag must be present, and the letter O that it may be omitted. Thus, in this example, every element except <line> must have a start-tag. Only the <poem> and <anthology> elements must have end-tags as well.

from 5 Defining SGML Document Structures: The DTD. Sometimes begin elements do not even need to be provided? Depends on the DTD.

Some HTML element types allow authors to omit end tags (e.g., the P and LI element types). A few element types also allow the start tags to be omitted; for example, HEAD and BODY. The HTML DTD indicates for each element type whether the start tag and end tag are required.

Some HTML element types have no content. For example, the line break element BR has no content; its only role is to terminate a line of text. Such empty elements never have end tags. The document type definition and the text of the specification indicate whether an element type is empty (has no content) or, if it can have content, what is considered legal content.

from HTML 4.01: 3.2 SGML constructs used in HTML. I do not believe this violates SGML. Since the previous 2 SGML examples show that elements can be defined such that the end tag is optional.

SGML provides a way for creating two hierarchies from the same data, however different dtds and "namespaces" need to be adhered to. example:

<(anthology)anthology>

<(p.anth)p.anth>

<(p.anth)page>

<!-- other titles and lines on this page here -->

<(anthology)poem><title>The SICK ROSE

<(anthology)stanza>

<line>O Rose thou art sick.

<line>The invisible worm,

</(p.anth)page>

<(p.anth)page>

<line>That flies in the night

<line>In the howling storm:

<(anthology)stanza>

<line>Has found out thy bed

<line>Of crimson joy:

<line>And his dark secret love

<line>Does thy life destroy.

</(anthology)poem>

<!-- rest of material on this page here -->

</(p.anth)page>

</(p.anth)p.anth)

</(anthology)anthology>

from 6 Complicating the Issue: More on Element Declarations. Does this violate the concept of well-formedness in XML? How does this relate to "overlapping?" More information at Chapter 10 Multiple Document Structures (SUBDOC, CONCUR and LINK) [from Web SGML and HTML 4.0 Explained]

"Web SGML and HTML 4.0 Explained" by Martin Bryan

• 3.5 The role of an SGML parser - lists what an SGML parser should do, including "identify the boundary of each piece of markup, and its relationship with preceding or following markup"
• Web SGML Adaptations lists 13 "extensions in Annex J and those in Annex K apply to an SGML declaration the minimum literal at the start of the definition is extended to read "ISO 8879:1986 (WWW)", where WWW stands for World Wide Web." including, "other new features" that talks about requirements for validity that are different from SGML.
• 5.2.5 Ambiguous content models and 5.2.6 Analysing content models describe that model must be defined so that it is not ambiguous and that content must be written so that it can be parsed without ambiguity. Rules for parsing documents are provided.
• 5.3 Using elements "tags can be omitted when their presence can be implied without ambiguity."
• The question is - is the HTML DTD a *good* DTD? Does it have all of the appropriate declarations, inclusions, exclusions, etc.? Why did we end up with such different implementations? Where did the ambiguity come from? Was it that folks were not following the HTML/SGML spec correctly? Were the developers of the user agents sloppy or was it the spec or was it the user agents catering to sloppy authors?
• 9.2 Tag omission - "This optional SGML feature allows tags to be omitted when their presence can be unambiguously implied by the program from the structure of the document declared in the document type definition."
• 9.2.2 End-tag omission - "End-tags can be omitted wherever the tag is followed by the end-tag of another currently open element (i.e. one started at a higher level than the current element) or when the tag is followed by an element, or data character, that is not a permitted part of the element's content model. End-tags can also be omitted if their presence can be implied by the end of an SGML document or subdocument."
• 9.2.3 Omitting tags "The end-tag for the title can be omitted because the <P> used to identify the start of the first paragraph in the section is not valid within the content model of the section title. As the section title can only consist of text the parsing program will automatically recognize that a </TITLE> tag should precede the first of the <P> tags." also "The two paragraph end-tags (</P>) have been omitted from the minimized version of the coded text for different reasons. At the end of the first paragraph the tag can be omitted because the content model for the paragraph element does not allow other paragraphs to be directly embedded within a paragraph. As soon as the parsing program sees the second <P> it knows it can infer the presence of the end-tag of the preceding paragraph." - is this clear in the HTML DTD?

$Date: 2005/10/18 15:24:05 $