A bunch of sites mention support in IE9, but I assume this was something that appeared in the Betas, or RC. It doesn't appear to be supported in IE9 final. Can someone confirm this?
No, placeholder text for form elements was never implemented for IE9. It is easily (and commonly) done with JavaScript.
Placeholder support was added in IE10.
来源：https://stackoverflow.com/questions/5329175/does-ie9-rtw-support-the-placeholder-attribute-on-input-elements