They can let you know if there are any errors present that might cause display issues and give you some clues for debugging.Validators do this by checking your code against a specification, or a set of rules for the language that your code was written in.The validator should indicate the line of code in your HTML that the error was found, so you can easily locate and fix the problem.The validator will display errors for all types of standard tags and elements (TD, TR, TABLE, DIV, A, STRONG, etc.) All HTML elements should have an appropriate closing tag to be considered valid, and having valid (and properly nested) closing tags are critically important with HTML email.The TITLE element is required in all HTML documents.In XHTML, unlike HTML, element and attribute names must be all lowercase.
Also, Campaign Monitor has written up a nice post about the use of DOCYTPE in email.
And one old browser even finds the entity §, converting §ion=2 to §ion=2.
To avoid problems with both validators and browsers, always use & in place of & when writing If you receive the error "Missing a required sub-element of HEAD", check that you have included the TITLE element in the HEAD.
They include tools for specific editors, too, such as Home Site, Front Page, Dreamweaver, others.
As mentioned in the HTML 4 Recommendation's note about specifying non-HTML data in element content, end tags are recognized within SCRIPT elements, but other kinds of markup--such as start tags and comments--are not.In Java Script, authors may use a backslash to prevent the string from being parsed as markup: , authors must also take care when using start tags within a script element.For details, see the Script and Style elements section of the XHTML 1.0 Recommendation as well as the HTML compatibility guideline for embedded scripts.One of the most popular validators is the free one maintained by the W3C, or World Wide Web Consortium.