事件处理程序 DOM事件处理程序 案例2: <script> function checkUsernameo( <fo rm me d p st action=http://www.example.org/register var elMsg document getElementByld(feedback,) if (this value length 5)( Label for="username">Create a elMsg textContent ' Username must be 5 username:</labe1> characters or more: 1 Input type="text id="username"/> else <div id="feedback">/div> eMsg textcontent= Label for= password">Create a password: </labeI> <input ty p password var elUsername document getElementByld id="password"/> Username) Input type="submit value="sig Username onblur= checkUsername /When it loses focus call checkuserNameO </script> K/form>事件处理程序 DOM事件处理程序  案例2：  < f o r m m e t h o d = " p o s t " action="http://www.example.org/register">  <label for="u sername">Creat e a username: </label>  <input type="text" id="username" />  <div id="feedback"></div>  <label for="p assword">Creat e a password: </label>  < i n p u t t y p e = " p a s s w o r d " id="password" />  <input type="submit" value="sign up" />  </form> <script> function checkUsername() { var elMsg = document.getElementById('feedback'); if (this.value.length < 5) { elMsg.textContent = 'Username must be 5 characters or more'; } else { elMsg.textContent = ''; } } var elUsername = document.getElementById ('username'); elUsername.onblur = checkUsername; // When it loses focus call checkuserName() </script>