SOURCE

document.addEventListener('DOMContentLoaded', () => {
  Vue.component('alert-box', {
    template: `
      <div class="demo-alert-box">
        <strong>Error!</strong>
        <slot></slot>
      </div>
    `
  });

  new Vue({
    el:'#app',
  });
});

<div id="app">
  35345
  <alert-box>134</alert-box>
  <alert-box>sdf34</alert-box>
</div>