The W3C has a page with explanations for many errors, which can be helpful.
A couple folks have created alternative validator tools that include warnings for HTML and/or CSS elements that aren’t supported by email clients.
HTML validators are invaluable tools for people that write code.
They can let you know if there are any errors present that might cause display issues and give you some clues for debugging.
An element is considered deprecated when it is slated for removal from the specification when new or better ways of doing the same thing are introduced.
For example, U (for underlining) was deprecated when CSS was introduced and it is now preferred to use the “text-decoration” property.
To be considered valid under an XHTML doctype, these tags require a trailing slash (/) at the end of the tag: In an HTML doctype, will produce a warning (usually “NET-enabling start-tag requires SHORTTAG YES”).
Both approaches work fine with HTML email, but if you’re concerned you can clean this up before re-validating your code.
Links that include web analytics or conversion tracking may trigger warnings such as “reference to entity for which no system identifier could be generated” or “cannot generate system identifier.” These are usually due to an unencoded ampersand present in the URL.
Curious as to how this can fit into your overall production workflow?
Use the results from the State of Email Production report to help benchmark your own process, identifying opportunities for improvement and using this evidence to make a compelling argument for more resources or process streamlining.
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.