Simply Testable Help and Documentation

Reference to entity "X" for which no system identifier could be generated
General entity "X" not defined and no default entity
End tag for "X" omitted, but omittag no was specified
Document type does not allow element "X" here; missing one of Y start tag
Attribute X not allowed on element Y at this point.
There is no attribute "X"
Required attribute "X" not specified
& did not start a character reference. (& probably should have been escaped as &.)
Element X not allowed as child of element Y in this context. (suppressing further errors from this subtree.)
Bad value W for attribute X on element Y: Z
Document type does not allow element "X" here
An img element must have an alt attribute, except under certain conditions. for details, consult guidance on providing text alternatives for images.
ID "X" already defined
Element "X" undefined
The X attribute on the Y element is obsolete. Use CSS instead.
End tag for element "X" which is not open
"X" is not a member of a group specified for any attribute
No X element in scope but a X end tag seen.
Net enabling start tag not immediately followed by null end tag
An attribute value specification must be an attribute value literal unless shorttag yes is specified
Value of attribute "X" must be a single token
Unclosed element X.
Duplicate id X.
End tag for X seen, but there were unclosed elements.
End tag for "X" which is not finished
The for attribute of the label element must refer to a form control.
The X element is obsolete. Use CSS instead.