In an ASP.net web application, using the "select" HTML control as server control and XHTML 1.1 validation, the "multiple" attribute in the markup will cause a parser error in application server if the value set to the multiple attribute is the "multiple" value (as dictates XHTML 1.1 validation standard).
When using the XHTML 1.1 validation, Visual Studio will even suggest the "multiple" value. If we set the value to "true" instead, the error disappears, but the value will be flagged by Visual Studio as a warning:
"Validation (XHTML 1.1): The values permitted for this attribute do not include 'true'".
Even so, the markup generated to the browser will be correct, using the "multiple" value instead.
One workaround is to add the attribute in the server side code.