title: Which characters are Invalid (unless encoded) in an XML attribute?
body: <p>I can't believe I can't find this information easily accessible, so:</p>
  
  <p>1) Which characters cannot be incorporated in an XML attribute without entity-encoding them?</p>
  
  <p>Obviously, you need to encode quotes. What about <code>&lt;</code> and <code>&gt;</code>? What else?</p>
  
  <p>2) Where exactly is the official list?</p>