Modal opens on page load - html

Question

so I have a modal on my site and it opens fine and everything but there is one problem ... it opens for a second then closes on page load. How can I fix this? It is powered by CSS and not JavaScript. The button you press to open the modal is included in my navigation bar.
Modal HTML:
<div id="JoinUs" data-toggle="modal" class="modalDialog">
<div>
X
<h4>Join Us!</h4>
<input type="text" id="fieldUser" class="input" required pattern=.*\S.* />
<label for="fieldUser" class="label">Username</label>
<input type="password" id="fieldPassword" class="input" required pattern=.*\S.* />
<label for="fieldPassword" class="label">Password</label>
<button class="btn">Login</button>
</div>
</div>
Modal CSS:
.modalDialog {
position: fixed;
font-family: Arial, Helvetica, sans-serif;
top: 0;
right: 0;
bottom: 0;
left: 0;
background: rgba(0,0,0,0.8);
z-index: 99999;
opacity:0;
-webkit-transition: opacity 400ms ease-in;
-moz-transition: opacity 400ms ease-in;
transition: opacity 400ms ease-in;
pointer-events: none;
}
.modalDialog:target {
opacity:1;
pointer-events: auto;
}
.modalDialog > div {
width: 400px;
position: relative;
margin: 10% auto;
padding: 5px 20px 13px 20px;
border-radius: 10px;
background: #fff;
background-color: #fff;
}
.close {
background: #606061;
color: #FFFFFF;
line-height: 25px;
position: absolute;
right: -12px;
text-align: center;
top: -10px;
width: 24px;
text-decoration: none;
font-weight: bold;
-webkit-border-radius: 12px;
-moz-border-radius: 12px;
border-radius: 12px;
-moz-box-shadow: 1px 1px 3px #000;
-webkit-box-shadow: 1px 1px 3px #000;
box-shadow: 1px 1px 3px #000;
}
.close:hover { background: #2ecc71; }