<input name="user_id" type="text" title="user id" onfocus="this.className='idOn';" onblur="if (!this.value) this.className='idOff'" class="idOff" />

CSS code

.idOff {border:1px solid #B8BCE2; background:#fff url(../images/default/login_id.gif) no-repeat 10px 3px;}
.idOn {border:1px solid #4e64f8;}