//占位符點擊消失
    <input type="text" id="text" value="請輸入一個數(shù)字">
    -----------------------------------
    var input_txt = document.getElementById("text");
    function clicknone() {
        if( input_txt.value == "請輸入一個數(shù)字" ) {
            input_txt.value = "";
        }

        if( input_txt.value == ""){
            input_txt.value = "請輸入一個數(shù)字";
        }
    }
    //獲得焦點
    input_txt.onfocus = function(){
        clicknone();
    }
    //失去焦點
    input_txt.onblur = function() {
        clicknone();
    }