This is the declaration of an xml document
<?xml version="1.0" encoding="utf-8"?>
An optional attribute "standalone" can be uses to identify whether an xml document need to reference an external reference.
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
DTD standard for Document Type Definition, it is a kind of define to verify whether an xml document is a valid xml document.
We can use a <!DOCTYPE> element to create DTD, like this
<!DOCTYPE demo [
<!ELEMENT demo (date, location)>
<!ELEMENT date (#PCDATA)>
<!ELEMENT location (#PCDATA)>
<!ENTITY kt "Kown Tong">
<!ENTITY klncity "Kowloon City">
Inside the Day 5 study, it seems not much related to the namespace handling in DTD. And, the NTATION, NTOKEN, MNTOKEN ... etc. need to have more explainations.