No < in Attribute Values |
Text from specification
The replacement text of any entity referred to directly or indirectly in an attribute value (other than "&lt;") must not contain a <.
Explanation
This restriction is meant to simplify the task of parsing XML data. Since attribute values can't even appear to contain element data, simple parsers need not track literal strings. Just by matching < and > characters, simple parsers can check for proper markup formation and nesting.