index.html

<style type="text/css">
  /* parent button containter css */
  h3,
  h4 {
    padding: 0;
    margin: 0;
  }
  #calendar-container {
    position: relative;
    width: 200px;
    height: 60px;
  }

  /* button css */
  #calendar {
    width: 200px;
    height: 60px;
  }

  /* Calendar layout */
  .calendar-layout {
    position: absolute;
    top: 60px;
    left: 0;
    background-color: yellow;
    width: 300px;
    height: 186px;
    display: none;
  }
  .calendar-layout-active {
    display: block;
  }

  .calendar-table {
    border: 1px solid red;
    width: 100%;
    border-collapse: collapse;
  }
  .calendar-table th {
    border: 1px solid red;
    text-align: center;
    padding: 4px;
  }
  .calendar-table td {
    text-align: center;
    font-weight: bold;
    border: 1px solid black;
    background-color: gray;
    padding: 4px;
    cursor: pointer;
  }
  .calendar-table td:hover {
    background-color: yellow;
    color: red;
  }
  .calendar-header {
    border-bottom: 1px solid red;
    padding-left: 4px;
    position: relative;
  }
  .calendar-header h3 {
    display: inline-block;
  }
  .calendar-table .btn-disabled {
    cursor: not-allowed;
  }
  .calendar-table .btn-disabled:hover {
    background-color: gray;
  }
  .month-nav-container {
    display: inline-block;
    position: absolute;
    right: 0;
  }
  .month-nav {
    width: 30px;
    height: 20px;
    margin: 1px 0px;
    font-weight: bold;
    font-size: 16px;
    line-height: 2px;
    font-family: monospace;
    cursor: pointer;
  }
  .prev-month {
    margin-right: 4px;
  }
  #current-selected-date {
    background-color: red;
    color: white;
    padding-left: 4px;
  }
  .calendar-table .each-day-active {
    background-color: yellow;
    font-weight: bold;
  }
</style>
<div id="app"></div>