custom_sidebar.html

<div id="mySidebar" class="sidebar">
    <a href="javascript:void(0)" class="closebtn" onclick="closeSidebar()">&times;</a>
    <a href="#">About</a>
    <a href="#">Services</a>
    <a href="#">Clients</a>
    <a href="#">Contact</a>
</div>

<div id="main">
    <button class="openbtn" onclick="openSidebar()">&#9776; Open Sidebar</button>
    <h1>Animated Sidebar Using  HTML, CSS, and JavaScript</h1>
    <div class="">
        Click the above button to open the sidebar menu.
    </div>
</div>

Per my comment on Medium for a11y improvement:

<div id="mySidebar" class="sidebar" aria-hidden="true">
    <button type="button" class="closebtn" onclick="closeSidebar()" aria-controls="mySidebar">&times;</a>
    <a href="#">About</a>
    <a href="#">Services</a>
    <a href="#">Clients</a>
    <a href="#">Contact</a>
</div>

<div id="main">
    <button type="button" class="openbtn" onclick="openSidebar()" aria-controls="mySidebar" aria-expanded="false">&#9776; Open Sidebar</button>
    <h1>Animated Sidebar Using  HTML, CSS, and JavaScript</h1>
    <div class="">
        Click the above button to open the sidebar menu.
    </div>
</div>