A Document Type Definition is a file that lists the behaviors of the various tags and attributes that make up the code for HTML and other markup languages. It essentially acts as a dictionary for the language, defining what tags exist and how they can be used. A parser is a program that checks to see that all necessary input has been provided. Logical checking examines the structure of document content to ensure accessibility . Unlike validation, which checks to see if a document is using markup correctly, Logical Checking looks to see if the document will actually provide enough information for all users to use the document.
Most Web browsers can recover from many errors in markup.
This is, of course, desirable because it means that even poorly written documents can be viewed on the World Wide Web. Correctly written code will render faster and with less problems than code with errors.