how to make a website mobile responsive with html and css code example

Example 1: responsive html

<meta name="viewport" content="width=device-width, initial-scale=1.0">

Example 2: make it responsive

<div id="gallery">
            <div class="showcase-images">
                <div class="showcase-block">
            <img id="show-img" src="images/showcase/tabs.png" alt="Tabs">
            </div>
            </div>
            <nav>
            <ul id="main">
                <li id="tabs"><a href="#">Tab</a></li>
                <li id="login"><a href="#">Login</a></li>
                <li id="forum"><a href="#">Forum</a></li>
            </ul>
        </nav>
        </div>