Examples of these errors are “end tag for TABLE omitted,” “end tag for element A which is not open,” or “end tag for P which is not finished.” These are critical, but easy to fix.
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.
However, there are a handful of elements that are considered empty elements or self-closing tags (namely BR and IMG in email).
For example, U (for underlining) was deprecated when CSS was introduced and it is now preferred to use the “text-decoration” property.The problem with using DOCTYPE with email is that some clients strip out the DOCTYPE or apply their own.If you don’t include a DOCTYPE in your HTML file, the W3C validator will use the HTML 4.01 Transitional Document Type. Generally speaking, I recommend using the HTML 4.01 Transitional or XHTML 1.0 Transitional when validating HTML for email.HTML validators work best for email when you use them to check for syntax errors, unclosed tags, or orphaned tags. HTML validation works by comparing your HTML to a set of standards or rules, called a DOCTYPE.When attempting to validate HTML coded for email with a web validator, it’s pretty common to see errors and warnings that are confusing. DOCTYPE is a document type declaration, and is usually placed at the beginning of your HTML file to tell the validator which set of standards to check your HTML against: In the case of web development, it also tells the browser which rendering mode to use.
Deprecated tags are still supported, and in some cases an email client may only support the older, deprecated element.