IE11浮动输入标签错误(占位符显示的问题？)

var input = document.getElementById('myInput');
input.addEventListener('focus', function() {
    if (input.value === '') {
        var placeholder = document.createElement('span');
        placeholder.className = 'placeholder';
        placeholder.innerHTML = '请输入内容';
        input.parentNode.insertBefore(placeholder, input);
    }
});
input.addEventListener('input', function() {
    var placeholder = input.parentNode.querySelector('.placeholder');
    if (input.value === '') {
        placeholder.style.display = 'inline';
    } else {
        placeholder.style.display = 'none';
    }
});

input[type="text"] {
    background-image: url('placeholder.png');
    background-position: 5px center;
    background-repeat: no-repeat;
    padding-left: 25px;
    color: #999999;
}