When I try to name my a4j:commandButton by the value <<
Like that  
<a4j:commandButton id="myButton" 
    value="<<"
    render="myGrid"
    styleClass="style_btn"
    disabled="false" />
There is an error occure
My debugging trace
Error Traced[line: 88] The value of attribute "value" associated with an element type "null" must not contain the '<' character.] with root cause
javax.faces.view.facelets.FaceletException: Error Parsing /screens/s1.xhtml: Error Traced[line: 88] The value of attribute "value" associated with an element type "null" must not contain the '<' character.
    at com.sun.faces.facelets.compiler.SAXCompiler.doCompile(SAXCompiler.java:390)
    at com.sun.faces.facelets.compiler.SAXCompiler.doCompile(SAXCompiler.java:364)
 
     
    