Function to clear placeholder text in an input element.

placholder.js

/**
 * Function to clear placeholder text in an input element.
 *
 * Clears the input on focus unless the user has already
 * entered data, if the input is blank on blur the placeholder
 * will be reinserted.
 *
 * Uses the value supplied in the _placeholder_ attribute on
 * the input element. If this is not present it will default 
 * to the _value_ attribute.
 *
 * NOTE: Implementation will default to the native browser
 * implementation if supported.
 */

function clearPlaceholder (field) {
 var placeholder = field.getAttribute('placeholder') || field.value;

 if (clearPlaceholder.supported === undefined) {
   clearPlaceholder.supported = ('placeholder' in document.createElement('input'));
 }

 if ( ! clearPlaceholder.supported) {
   field.value = field.value || placeholder;
   field.onfocus = function () {
     if (this.value === placeholder) {
       this.value = '';
     }
   };
   field.onblur = function () {
     if (this.value === '') {
       this.value = placeholder;
     }
   };
 }
}

// Call when document has loaded.
// Example Input Element: <input type="email" name="email" placeholder="[email protected]" value="" />
var input = document.getElementById('signup').getElementsByTagName('input')[0];
clearPlaceholder(input);