Using JQUERY to achieve all selection and cancel all selection, not as cumbersome as js, and support more browsers.

<mce:script type="text/javascript"><!--

$(function() { 
    $("#checkall").click(function() { 
        $("input[@name='checkname[]']").each(function() { 
            $(this).attr("checked", true);
        });
    });
    $("#delcheckall").click(function() { 
        $("input[@name='checkname[]']").each(function() { 
            $(this).attr("checked", false);
        });
    });
});
// --></mce:script>

<input type='checkbox' id='id1' name='checkname[]' value='1' />value1 
<input type='checkbox' id='id2' name='checkname[]' value='2' />value2
<input type='checkbox' id='id3' name='checkname[]' value='3' />value3

<input type="button" id="checkall" name="checkall" value=" Future generations " />
<input type="button" id="delcheckall" name="delcheckall" value=" To cancel all " />

A simpler one:

$("#checkall").click(
  function(){
    if(this.checked){
        $("input[name='checkname']").attr('checked', true)
    }else{
        $("input[name='checkname']").attr('checked', false)
    }
  }
);