Untitled

#button {
    display: block;
    color: rgb(255, 255, 255);
    text-decoration: none;
    text-align: center;
    padding: 10px;
    margin: 15px 0px 0px 15px;
    font-size: 18px;
    text-transform: uppercase;
    background: #ED5E2F;
    color: #FFF;
    border: 0px none;
    outline: 0px none;
	top:0;
	left:0;
	position:fixed;
	z-index:100;
}

#button:hover {
    background: #74A599;
}

#button:active {
    background: #F6A953;
}
#witaj
{
color:white;
text-align:center;
background-color:#545A5C;
width:400px;
height:40px;
top:15;
left:40;
position:fixed;
z-index:100px;
}

.hidden{
    display: none;
}

$(document).ready(function(){
    $("#button").click(function(){
        $("#witaj").hide();
    });
});

window.addEventListener('DOMContentLoaded', function(){

    document.getElementById('button').addEventListener('click', function(){

        document.getElementById('witaj').classList.add('hidden');

    });

});

<html>
<head>
<link rel="stylesheet" type="text/css" href="style.css">
<script type="text/javascript" src="script.js"></script>
<script src="//code.jquery.com/jquery-1.11.3.min.js"></script>
<body>
<div id="witaj">
<button id="button">OK</button>
</div>

</body>


<head>
</html>