Your DTD looks pretty good. I only see a few things.
An element can't have a space in the name, so you'll have to change aircraft fleets
. There's no reason you couldn't shorten it to fleet
.
For readability, it's good practice to put the attribute declaration (ATTLIST
) under the element declaration so it's easy to find. You also only need one ATTLIST
per element. You can add linebreaks and tabs/spaces to make it easy to read...
<!ELEMENT airport (hanger+)>
<!ATTLIST airport
name CDATA #REQUIRED
location CDATA #REQUIRED
famous_assoiciation CDATA #IMPLIED >
The color
attribute is supposed to have a default value of "silver", but you have it as a fixed value. Just remove #FIXED
...
<!ATTLIST plane color CDATA "silver">