placeholder focus 할때 없애기

input:focus::-webkit-input-placeholder, textarea:focus::-webkit-input-placeholder { /* WebKit browsers */ color:transparent; } 

input:focus:-moz-placeholder, textarea:focus:-moz-placeholder { /* Mozilla Firefox 4 to 18 */ color:transparent; } 

input:focus::-moz-placeholder, textarea:focus::-moz-placeholder { /* Mozilla Firefox 19+ */ color:transparent; } 

input:focus:-ms-input-placeholder, textarea:focus:-ms-input-placeholder { /* Internet Explorer 10+ */ color:transparent; }