As the four-part list suggests, this is actually a bit on the tricky side. But it's another area where all the tricky bits have been assigned to the XML Processor so as to make the application's life easier.
Attribute values are normally short and simple, except when they're not. The idea here is that you can generate XML that looks like this:
<!ENTITY home 'xml.com'>
<El At="
found at
&home;
">
Then all the application will see for that attribute value is
found at xml.com.