<script>
var doc = document,
    inputs = doc.getElementsByTagName('input'),
    supportPlaceholder = 'placeholder' in doc.createElement('input'),
    placeholder = function(input){
        var text = input.getAttribute('placeholder'),
            defaultValue = input.defaultValue;

        input.value = text;
        input.onfocus = function(){
            if(input.value === defaultValue || input.value === text){
                this.value = '';
            }
        }
        input.onblur = function(){
            if(input.value === ''){
                this.value = text;
            }
        }
    };

if(!supportPlaceholder){
    for(var i = 0, len = inputs.length; i < len; i++){
        var input = inputs[i], text = input.getAttribute('placeholder');
        if(input.type === 'text' && text){
            placeholder(input);
        }
    }
}
</script>

学习笔记：

• 连续 var
• 检测浏览器支持 supportPlaceholder = ‘placeholder’ in doc.createElement(‘input’)

via 跨浏览器 HTML5 Placeholder

但是如果有 默认值的话，似乎有 bug。一旦获得焦点，defaultValue 就被强制换成 placeholder 了，改了一个版本（为了方便chrome调试，placeholder -> placeholder2 了；用的时候改回来）

<input placeholder2="123" value="88888" />

<script>
var doc = document,
    inputs = doc.getElementsByTagName('input'),
    supportPlaceholder = 'placeholder2' in doc.createElement('input'),
    placeholder = function(input){
        var text = input.getAttribute('placeholder2'),
            defaultValue = input.defaultValue;

                 if(!defaultValue){
            input.value = text;
          }        

        input.onfocus = function(){
            if( input.value === text){
                this.value = '';
            }
        }
        input.onblur = function(){
            if(input.value === ''){
                this.value = text;
            }
        }
    };

if(!supportPlaceholder){
    for(var i = 0, len = inputs.length; i < len; i++){
        var input = inputs[i], text = input.getAttribute('placeholder2');
        if(input.type === 'text' && text){
            placeholder(input);
        }
    }
}
</script>