Rootdiv

class bootstrap_4_walker_nav_menu extends Walker_Nav_menu {
    
    function start_lvl( &$output, $depth = 0, $args = array() ){ // ul
        $indent = str_repeat("\t",$depth); // indents the outputted HTML
        $submenu = ($depth > 0) ? ' sub-menu' : '';
        $output .= "\n$indent<ul class=\"dropdown-menu$submenu depth_$depth\">\n";
    }
  
  function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ){ // li a span
        

Rootdiv

<div id="app">
	<div class="container">
		<h1 class="title">{{ title }}</h1>
		<div class="card">
			<div class="card__inner">
				<form>
					<input type="text" placeholder="Create a new todo" v-model="newItem" />
					<button @click="addItem" :disabled="newItem.length === 0">Add Todo</button>
				</form>
				<p v-if="items.length === 0">No todos!</p>