<...> lacks "..." attribute

Cause:

An HTML tag has a mandatory attribute. The attribute is missing.

Solution:

Consult the tag definition. Add the missing attribute.

Sample:

<form> lacks "action" attribute

BAD :  <form>
GOOD:  <form action="my_action.jsp">


References:

W3C: entities: http://www.w3.org/TR/html4/index/elements.html