Mixed Content

<!ELEMENT PRODUCT (#PCDATA | PRODUCT_ID)*>                    

<PRODUCT>Tomatoes</PRODUCT>

<PRODUCT>
   <PRODUCT_ID>124829548702121</PRODUCT_ID>
</PRODUCT>

<!ELEMENT p (#PCDATA | b)*>
<!ELEMENT b (#PCDATA)>

<p>This is <b>bold</b> text</p>