Chapter 3. Well-Formedness
The very first step in moving markup into modern form is to make it well-formed. Well-formedness is the basis of the huge and incredibly powerful XML tool chain. Well-formedness guarantees a single unique tree structure for the document that can be operated on by the DOM, thus making it the basis of reliable, cross-browser JavaScript. The very first thing you need to do is make your pages well-formed.
Validity, although important, is not nearly as crucial as well-formedness. There are often good reasons to compromise on validity. In fact, I often deliberately publish invalid pages. If I need an element the DTD doesn’t allow, I put it in. It won’t hurt anything because browsers ignore elements they don’t understand. ...
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