1. Reference to entity "X" for which no system identifier could be generated

  2. General entity "X" not defined and no default entity

  3. End tag for "X" omitted, but omittag no was specified

  4. Document type does not allow element "X" here; missing one of Y start tag

  5. Attribute X not allowed on element Y at this point.

  6. There is no attribute "X"

  7. Required attribute "X" not specified

  8. & did not start a character reference. (& probably should have been escaped as &.)

  9. Element X not allowed as child of element Y in this context. (suppressing further errors from this subtree.)

  10. Bad value W for attribute X on element Y: Z

  11. Document type does not allow element "X" here

  12. An img element must have an alt attribute, except under certain conditions. for details, consult guidance on providing text alternatives for images.

  13. ID "X" already defined

  14. Element "X" undefined

  15. The X attribute on the Y element is obsolete. Use CSS instead.

  16. End tag for element "X" which is not open

  17. "X" is not a member of a group specified for any attribute

  18. No X element in scope but a X end tag seen.

  19. Net enabling start tag not immediately followed by null end tag

  20. An attribute value specification must be an attribute value literal unless shorttag yes is specified

  21. Value of attribute "X" must be a single token

  22. Unclosed element X.

  23. Duplicate id X.

  24. End tag for X seen, but there were unclosed elements.

  25. End tag for "X" which is not finished

  26. The for attribute of the label element must refer to a form control.

  27. The X element is obsolete. Use CSS instead.