I am having the codes in below, however, when I modify the code with visual studio code and run it in my Chrome browser, I cannot see the drop down menu in the "Hot Products." section. 

However, when I insert the code in here (stackoverflow), I can see my drop down menu. Can someone explain to me how come this happened? And how can I fix this problem so that I can view that in all types of browsers? Thanks a lot!

```css
body {
    background-color: white;
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-size: cover;
}

/* Navigation bar */
#navigation_bar {
    list-style-type: none;
    margin: 0;
    padding: 0;
    overflow: hidden;
    background-color: #333;
    position: fixed;
    top: 0;
    width: 100%;
}

li {
    float: left;

}

li a, .dropbtn {
    display: inline-block;
    color: white;
    text-align: center;
    padding: 14px 16px;
    text-decoration: none;
}

li a:hover:not(.active), .dropdown:hover .dropbtn {
    background-color: #111;
}

li.dropdown {
    display: inline-block;
}

.dropdown-content {
    display: none;
    position: absolute;
    background-color: #4CAF50;
    min-width: 160px;
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
}

.dropdown-content a {
    color: black;
    padding: 12px 16px;
    text-decoration: none;
    display: block;
    text-align: left;
}

.dropdown-content a:hover {
    background-color: #f1f1f1
}

.dropdown:hover .dropdown-content {
    display: block;
}

.active {
    background-color: #4CAF50;
}
```

```html
<!DOCTYPE html>
<html>
    <head>
        <title>Bo Kei Restaurant</title>
        <link rel='stylesheet' type='text/css' href='stylesheet.css'/>
        <script type='text/javascript' src='javascript/jquery-3.1.0.js'></script>
        <script type='text/javascript' src='javascript/jquery-ui.min.js'></script>
        <script type='text/javascript' src='javascript/script.js'></script>
    </head>
    <body>

<!--Navigation bar-->
        <ul id="navigation_bar">
            <li><a class="active" href="navigation_bar/home.html">Home.</a></li>
            <li><a href="navigation_bar/promotion.html">Promotion.</a></li>
<!--drop down menu-->
           <li class="dropdown"><a href="navigation_bar/hot_products.html" class="dropbtn">Hot Products.</a>
                <div class="dropdown-content">
                    <a herf="navigation_bar/sub_menu/sandwiches.html">Sandwiches</a>
                    <a herf="navigation_bar/sub_menu/burger.html">Burger</a>
                    <a herf="navigation_bar/sub_menu/rice.html">Rice</a>
                    <a herf="navigation_bar/sub_menu/noodles.html">Noddles</a>
                </div>
            </li>   
<!--Back to normal-->
            <li><a href="navigation_bar/cold_products.html">Cold Products.</a></li>
            <li><a href="navigation_bar/snacks.html">Snacks.</a></li>
            <li><a href="navigation_bar/about_us.html">About Us.</a></li>
            <li><a href="navigation_bar/contact_us.html">Contact Us.</a></li>
        </ul>

    </body>
</html>
```