Add <h1> tag inside the <li> tag on current menu item: this walker class will check if the item url is the current page url, and if so will add the <h1> tag. Useful for SEO purpose.

custom_walker_nav_menu.php

class custom_walker_nav_menu extends Walker_Nav_Menu {
  
  
	// add main/sub classes to li's and links
	 function start_el( &$output, $item, $depth, $args ) {
	    global $wp_query;
	    $indent = ( $depth > 0 ? str_repeat( "\t", $depth ) : '' ); // code indent
	  
	    // depth dependent classes
	    $depth_classes = array(
	        ( $depth == 0 ? 'main-menu-item' : 'sub-menu-item' ),
	        ( $depth >=2 ? 'sub-sub-menu-item' : '' ),
	        ( $depth % 2 ? 'menu-item-odd' : 'menu-item-even' ),
	        'menu-item-depth-' . $depth
	    );
	    $depth_class_names = esc_attr( implode( ' ', $depth_classes ) );
	  

	    // passed classes
	    $classes = empty( $item->classes ) ? array() : (array) $item->classes;
	    $class_names = esc_attr( implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) ) );
	  
	    // build html
	    $output .= $indent . '<li id="nav-menu-item-'. $item->ID . '" class="' . $depth_class_names . ' ' . $class_names . '">';
	  
	    // link attributes
	    $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
	    $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
	    $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
	    $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
	    $attributes .= ' class="menu-link ' . ( $depth > 0 ? 'sub-menu-link' : 'main-menu-link' ) . '"';
	  

	  	// check if item url is current page url
	  	$permalink = get_permalink();
	  	if ($item->url == $permalink) {
	  		$markup_start = "<h1 class='font-size__m margin-vert--null no-decoration'>";
	  		$markup_end = "</h1>";

	  	} else {
	  		$markup_start = "";
	  		$markup_end = "";
	  	}

	    $item_output = sprintf( $markup_start . '%1$s<a%2$s>%3$s%4$s%5$s</a>%6$s' . $markup_end,
	        $args->before,
	        $attributes,
	        $args->link_before,
	        apply_filters( 'the_title', $item->title, $item->ID ),
	        $args->link_after,
	        $args->after
	    );

	  
	    // build html
	    $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );

	}
}