Please take a look at this example in Chrome browser: http://jsfiddle.net/ymzTB/
<form>
<input required type = "text" >
<input type = "submit" >
</form>
When you leave the input empty and just click enter, you'll receive a message like this: Please fill out this field.
which looks nice.
How can I style this message in WebKit and other browsers, say, replace the background color with red, or set the font color or size to something else?