Inténtalo tú mismo - W3.CSS examples_modal

<!DOCTYPE html>
<html>
<title>W3.CSS</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="/lib/w3.css">
<body class="w3-container">

<h2>W3.CSS Login Modal</h2>

<button onclick="document.getElementById('id01').style.display='block'" class="w3-btn w3-green w3-large">Login</button>

<div class="w3-container">
      <div class="w3-section">
        <label><b>Username</b></label>
        <input class="w3-input w3-border w3-hover-border-black w3-margin-bottom" type="text" placeholder="Enter Username">

<label><b>Password</b></label>
        <input class="w3-input w3-border w3-hover-border-black" type="text" placeholder="Enter Password">
        
        <button class="w3-btn w3-btn-block w3-green w3-section">Login</button>
        <input class="w3-check" type="checkbox" checked="checked"> Remember me
      </div>
    </div>

<div class="w3-container w3-border-top w3-padding-hor-16 w3-light-grey">
      <button onclick="document.getElementById('id01').style.display='none'" type="button" class="w3-btn w3-red">Cancel</button>
      <span class="w3-right w3-padding w3-hide-small">Forgot <a href="#">password?</a></span>
    </div>

</div>
</div>
            
</body>
</html>