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The WDG HTML Validator is similar in many ways to the W3C HTML Validation Service. Most of the previous differences between the two validators have disappeared with recent development of the W3C validator. The errors reported are the same in virtually all cases. However, some minor differences may arise:
The WDG HTML Validator gives warnings for valid but dangerous HTML: unclosed start-tags (e.g., <p><img src=foo alt=bar</p>), unclosed end-tags (e.g., <p><em>foo</em</p>), empty start and end-tags (e.g., <p>foo</>), and net-enabling start-tags (e.g., <img alt=foo src=bar/baz>). Other validators do not give such warnings.
The WDG HTML Validator gives warnings for undefined references such as “ and ™. The W3C HTML Validation Service reports such undefined references as errors for XML documents but remains quiet for HTML documents.
Unlike other validators, the WDG HTML Validator uses a special SGML declaration when the document refers to a custom DTD. The result is that many custom DTDs, especially those built from the HTML 4.0 Transitional DTD, work correctly with the WDG HTML Validator but not other validators.
The two validators differ significantly from other checking tools such as Weblint and CSE 3310 HTML Validator. (Despite its name, the latter is not a true "validator.") The two online validators check documents against a document type definition--a published, machine-readable document that facilitates an objective check of syntax against HTML standards. On the other hand, lints are subjective; they provide a more cursory syntax check while adding stylistic checks, such as warning about poorly supported features. Lints are useful tools, but they are not substitutes for HTML validation.