<ul>
  <li id='signUp'>Sign Up</li>
  <li id="logIn">Log In</li>
</ul>

<div id="logInModal" class="modal">
    <div class="modal-content">
        sssssss
    </div>
</div>

<div id="signUpModal" class="modal">
    <div class="modal-content">
        sssssss
    </div>
</div>

.modal {
    display: none;
    position: fixed;
    z-index: 1;
    padding-top: 100px;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    overflow: auto;
    background-color: rgb(0,0,0);
    background-color: rgba(0,0,0,0.4);
}

.modal-content {
    background-color: #fefefe;
    margin: auto;
    padding: 20px;
    border: 1px solid #888;
    width: 80%;
}

var logInModal = document.getElementById('logInModal');
    var logInButton = document.getElementById("logIn");

    logInButton.onclick = function() {
        logInModal.style.display = "block";
    }

    window.onclick = function(event) {
        if (event.target == logInModal) {
            logInModal.style.display = "none";
        }
    }

    var signUpModal = document.getElementById('signUpModal');
    var signUpButton = document.getElementById("signUp");

    signUpButton.onclick = function() {
        signUpModal.style.display = "block";
    }

    window.onclick = function(event) {
        if (event.target == signUpModal) {
            signUpModal.style.display = "none";
        }
    }