The validation process and building DTDs are the two most difficult parts of the XML life cycle.Briefly a DTD defines all the possible elements to be found within your document, what is the formal shape of your document tree (by defining the allowed content of an element; either text, a regular expression for the allowed list of children, or mixed content i.e. The DTD also defines the valid attributes for all elements and the types of those attributes.
However, HTML 4 Strict documents may look bland on very old browsers that lack support for style sheets. HTML 4.01 Strict is a trimmed down version of HTML 4.01 that emphasizes structure over presentation.Deprecated elements and attributes (including most presentational attributes), frames, and link targets are not allowed in HTML 4 Strict.Now that you’ve defined the custom attributes, how do you place them where a validator can find them?The very best place to put them would be as the and adding the new attribute specifications at the end of the file.
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that will add those custom attributes, and will show you how to validate documents that use those new attributes.