XML DTD constraints

< Element 1 > 
    < element 2 > 
        < element 3 > Description 1 </ element 3 > 
        < element 4 > Description 2 </ element 4 > 
    </ element 2 > 
</ element 1 >

1. DTD introduced on the network

<-! Dtd dtd path name on the document type root tag name of the network dtd ->
<! DOCTYPE stus the PUBLIC "// UNKNOWN /" "unknown.dtd">

2. The introduction of local DTD

<- introducing local DTD:! Root tag name location of the local introduction of the DTD dtd ->

<!DOCTYPE aaa SYSTEM "aaa.dtd">

3. embedded directly in the XML DTD inside constraint rules

<! - xml document constraint rules embedded directly inside the DTD ->
! <DOCTYPE stus [
　　! <1 the ELEMENT element (element 2)>
　　! <The ELEMENT element 2 (element 3, the element. 4)>
　　! <The ELEMENT element 3 ( #PCDATA)>
　　<! the eLEMENT element. 4 (#PCDATA)>
]>