Modal Login Form
<script>
// Get the modal
var modal = document.getElementById('id01');
// When the user clicks anywhere outside of the modal, close it
window.onclick = function(event) {
if (event.target == modal) {
modal.style.display = "none";
}
}
</script>
Şuradan detaylarına bakabilirsiniz
https://www.w3schools.com/howto/tryit.asp?filename=tryhow_css_login_form_modal