Keep in mind that some web browsers, such as MSIE 8, Firefox and Opera, don't currently support the placeholder attribute, but hopeful with another update or two, they will include that support. Either way, using it won't mess up the page, as those browsers will just ignore it anyway. Here is an example of what the placeholder attribute looks like in action:
For those of you not using compliant browsers, here is a graphic that shows it in action using the Chrome browser:
And here is the HTML5 code that is used to achieve that effect:
<input name="email" type="email" placeholder="email@example.com" />
This is just one example of how the use functionality of HTML5 will simplify the development of web pages that are more user-friendly. Stay tuned for other such examples!