Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE catalog [

<!ELEMENT catalog (product*, order*)>
    <!ELEMENT product (name, description?)>
    <!ELEMENT order   (lineItem+)>
    <!ELEMENT lineItem EMPTY>
    <!ELEMENT name        (#PCDATA)>
    <!ELEMENT description (#PCDATA)>

<!ATTLIST product
        id       ID                    #REQUIRED
        status   (active|discontinued) "active"
        sku      NMTOKEN               #IMPLIED
    >

<!ATTLIST lineItem
        productRef IDREF  #REQUIRED
        quantity   CDATA  #REQUIRED
        unit       (each|kg|litre) "each"
    >

<!ATTLIST catalog
        xmlns    CDATA   #FIXED "http://example.com/catalog"
        version  CDATA   #FIXED "1.0"
    >

]>
<catalog>
    <product id="p001" sku="WH-100">
        <name>Wireless Headphones</name>
    </product>
    <product id="p002" status="discontinued" sku="SP-200">
        <name>Bluetooth Speaker</name>
        <description>Portable speaker, waterproof.</description>
    </product>