_s theme navigation Javascript

nav-styles.css

/* Small menu */
.menu-toggle {
	display: none;
	cursor: pointer;
}

@media screen and (max-width: 600px) {
	.menu-toggle,
	.main-navigation.toggled .nav-menu {
		display: block;
	}

	.main-navigation ul {
		display: none;
	}
}

navigation.js

/**
 * navigation.js
 *
 * Handles toggling the navigation menu for small screens.
 */
( function() {
	var container, button, menu;

	container = document.getElementById( 'site-navigation' );
	if ( ! container )
		return;

	button = container.getElementsByTagName( 'h1' )[0];
	if ( 'undefined' === typeof button )
		return;

	menu = container.getElementsByTagName( 'ul' )[0];

	// Hide menu toggle button if menu is empty and return early.
	if ( 'undefined' === typeof menu ) {
		button.style.display = 'none';
		return;
	}

	if ( -1 === menu.className.indexOf( 'nav-menu' ) )
		menu.className += ' nav-menu';

	button.onclick = function() {
		if ( -1 !== container.className.indexOf( 'toggled' ) )
			container.className = container.className.replace( ' toggled', '' );
		else
			container.className += ' toggled';
	};
} )();

s-theme-header-markup.php

<header id="masthead" class="site-header" role="banner">
	<div class="site-branding">
		<h1 class="site-title"><a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></h1>
		<h2 class="site-description"><?php bloginfo( 'description' ); ?></h2>
	</div>

	<nav id="site-navigation" class="main-navigation" role="navigation">
		<h1 class="menu-toggle"><?php _e( 'Menu', '_s' ); ?></h1>
		<div class="screen-reader-text skip-link"><a href="#content" title="<?php esc_attr_e( 'Skip to content', '_s' ); ?>"><?php _e( 'Skip to content', '_s' ); ?></a></div>

		<?php wp_nav_menu( array( 'theme_location' => 'primary' ) ); ?>
	</nav><!-- #site-navigation -->
</header><!-- #masthead -->