How to close responsive menu by clicking an item and changing the iframe in an html file?

Question

I’m using a file index.html that on a wide screen appears all menu items, but when resized to a smaller screen (max-width: 640px), the menu bar becomes an icon where when clicked appears the menu items on the smaller screen.

My question is: after clicking on an item I would like the smaller screen menu to close and at the same time load an html file into iframe.

Note: This html file that will be loaded inside the iframe is corresponding to the menu item clicked.

Below is the code as an example:

body {
  margin: 0;
  padding: 0;
}

.menu {
  width: 100%;
  height: 50px;
  background-color: #bbb;
  font-family: 'Arial';
}

.menu ul {
  list-style: none;
  position: relative;
}

.menu ul li {
  width: 150px;
  float: left;
}

.menu a {
  padding: 15px;
  display: block;
  text-decoration: none;
  text-align: center;
  background-color: #c9c9c9;
  color: #fff;
}

.menu a:hover {
  background: #f4f4f4;
  color: #555;
}

label[for="bt_menu"] {
  padding: 5px;
  background-color: #a3a3a3;
  color: #fff;
  font-family: 'Arial';
  text-align: center;
  font-size: 30px;
  cursor: pointer;
  width: 50px;
  height: 50px;
}

label[for="bt_menu"] {
  display: none;
}

@media(max-width: 640px) {
  label[for="bt_menu"] {
    display: block;
  }
  #bt_menu:checked~.menu {
    margin-left: 0;
    position: absolute;
    visibility: visible;
    display: block;
  }
  .menu ul {
    margin: 0px;
    padding: 0px;
    width: 100%;
    position: absolute;
    visibility: visible;
  }
  .menu {
    margin-top: 5px;
    margin-left: -100%;
    transition: all .2s;
  }
  .menu ul li {
    width: 100%;
    float: none;
  }
  .icon {
    text-align: center;
    background-color: #e0e0e0;
    width: 100px;
    cursor: pointer;
    padding: 7px;
    border-radius: 4px;
    -webkit-border-radius: 4px;
    -moz-border-radius: 4px;
    -ms-border-radius: 4px;
    -o-border-radius: 4px;
  }
}

<input type="checkbox" id="bt_menu">
<label for="bt_menu" class="icon">Menu</label>

<nav class="menu">
  <ul>
    <li><a href="#" target="frame">Um</a></li>
    <li><a href="#" target="frame">Dois</a>
    <li><a href="#" target="frame">Tres</a></li>
    <li><a href="#" target="frame">Quatro</a></li>
  </ul>
</nav>

<iframe src="#" name="frame" frameborder="no" scrolling="no"></iframe>

Answer 1

I solved with this javascript

function fechar() {

    var inputCheckbox = document.getElementById('bt_menu');

    if (inputCheckbox.checked === true) {
        inputCheckbox.checked = false;
    }

}

Within the tags <a></a>, insert the event onclick="fechar()" to call the function.

Worked!