模态视图不尊重z-index规则，为什么？

Question

我使用了一些源代码，它在click事件中显示了一个模态视图框。在在线演示中，它运行良好，但在我的网页上，他的框加载有点扭曲，模式框位于网站徽标和网站标题文本后面。

我到目前为止在网上遇到的文章告诉我尝试z-index方法，并将所有涉及的元素设置为相对，但是这没有用，因为它进一步扭曲了我的其余页面元素。

我在这个jsFiddle：https://jsfiddle.net/a0ht7y51/

中复制了这个问题

此处也张贴相关代码：

HTML：

 <div id="header">

                    <img id="headerBackground" src="siteImages/tiltchair.jpg">

                    <img id="Logo" onclick="document.getElementById('id01').style.display='block'" align="middle" src="siteImages/final.png" alt="Logo">

                    <img id="Text" align="middle" src="siteImages/TWText.png" alt="Logo">

              <form class="modal-content animate" action="action_page.php">
                <div class="imgcontainer">
                  <span onclick="document.getElementById('id01').style.display='none'" class="close" title="Close Modal">&times;</span>
                </div>

                <div class="container">
                  <label><b>Username</b></label>
                  <input type="text" class="input-txt"  placeholder="Enter Username" name="uname" required>

                  <label><b>Password</b></label>
                  <input type="password" class="input-txt"  placeholder="Enter Password" name="psw" required>

                  <button type="submit">Login</button>
                  <input type="checkbox" checked="checked"> Remember me
                </div>


                  <span class="psw">Forgot <a href="#">password?</a></span>

              </form>

CSS：

.input-txt{
  width: 100%;
  padding: 20px 10px;
  background: $blue;
  border: none;
  font-size: 1em;
  color: white;
  border-bottom: 1px dotted rgba(250, 250, 250, .4);
  @include box-sizing(border-box);
  @include placeholder(lighten($blue, 10%));
  @include transition(background-color .5s ease-in-out);
   &:focus{
     background-color: darken($blue, 30%);
    }
}



p{

    font-size:16px 
    text-align: left;
}

h1 {

    font-size:16px
}

/* CSS for modal */

/* Full-width input fields */
input[type=text], input[type=password] {
width: 33%;
padding: 12px 20px;
margin: 8px 0;
display: block;
border: 1px solid #ccc;
    box-sizing: border-box;
    align-items: center;
}

/* Set a style for all buttons */
button {
    background-color: #1dacf9;
    color: white;
    padding: 1px 2px;
    margin: 8px 0;
    border: none;
    cursor: pointer;
    width: 33%;
}

/* Extra styles for the cancel button */
.cancelbtn {
    width: auto;
    padding: 10px 18px;
    background-color: #f44336;
}

 /* Center the image and position the close button */
.imgcontainer {
    text-align: center;
    margin: 24px 0 12px 0;

}

img.avatar {
    width: 40%;
    border-radius: 50%;
}

.container {
    padding: 6px;
}

span.psw {
    float: right;
    padding-top: 16px;
}   

/* The Modal (background) */
.modal {
    display: none; /* Hidden by default */
    position: fixed; /* Stay in place */
    z-index: 10; /* Sit on top */
    left: 0;
    top: 0;
    width: 100%; /* Full width */
    height: 100%; /* Full height */
    overflow: auto; /* Enable scroll if needed */
    background-color: rgb(0,0,0); /* Fallback color */
    background-color: rgba(0,0,0,0.4); /* Black w/ opacity */
    padding-top: 60px;
    height: 100vh;
    border-radius: 20px;
    }

 /* Modal Content/Box */
.modal-content {
    background-color: #fefefe;
    margin: 5% auto 15% auto; /* 5% from the top, 15% from the bottom and centered */
    border: 1px solid #888;
    width: 66%; /* Could be more or less, depending on screen size */
    height: 66vh;
    border-radius: 20px;

}

 /* The Close Button (x) */
.close {
     position: relative;
     margin: 2% auto 95% auto;
    color: #000;
    font-size: 35px;
    font-weight: bold;
}

.close:hover,
.close:focus {
    color: red;
    cursor: pointer;
}

 /* Add Zoom Animation */
.animate {
     -webkit-animation: animatezoom 0.6s;
     animation: animatezoom 0.6s
 }

 @-webkit-keyframes animatezoom {
    from {-webkit-transform: scale(0)} 
    to {-webkit-transform: scale(1)}
 }

@keyframes animatezoom {
    from {transform: scale(0)} 
    to {transform: scale(1)}
}  

 /* Change styles for span and cancel button on extra small screens */
@media screen and (max-width: 300px) {
     span.psw {
        display: block;
       float: none;
    }
    .cancelbtn {
       width: 100%;
    }
}



#headerBackground {

    position: fixed;
    width: 100%;
    top: 0;
    left: 0;
    height: 100vh;
    z-index: -10;
}

#TWLogo {

transform: translateZ(1px);
display: block;
margin-top: 5%;
margin: 0 auto;
cursor: pointer;
background-attachment: fixed;
padding-top: 50px;
padding-bottom: 50px;
width:150px;
height:125px;
transition: all .2s ease-in-out;
background-position:center;
z-index: -1;



}
#Logo:hover { transform: scale(1.1); }

#Text {


    display: block;
    margin: 0 auto;
    padding-top: 80px;
    padding-bottom: 50px;
    width:355px;
    height:125px;
    transform: translateZ(30px);
    background-position:center;
    z-index: -1;
}

所以我对CSS的最低限度的知识阻止了我看到什么是一个简单的解决方案，谁能看到它？