Why background-color overlap the box-shadow?

I have a table with cells that have a background-color. I'm trying to set the box-shadow for the thead, but the background of a cells in first column overlaps a shadow. I tried to set a different z-index values and position: relative, but to no avail.

.results-table {
    width: 100%;
    border-collapse: separate;
    border-spacing: 0;
    table-layout: fixed;
}

.results-table th,
.results-table td {
    padding: 1em;
    width: 96px;
    border-left: 1px solid rgba(34, 36, 38, 0.1);
    border-top: 1px solid rgba(34, 36, 38, 0.1);
}

.results-table thead {
  box-shadow: 0 2px 2px 0 rgba(0,0,0,.14), 0 3px 1px -2px rgba(0,0,0,.2), 0 1px 5px 0 rgba(0,0,0,.12);
  position: relative;
}

.results-table thead th {
    background: #E0E0E0;
    border-bottom: 1px solid rgba(34, 36, 38, 0.1);
}

.results-table tr:first-child td,
.results-table tr:first-child th {
    border-top: none;
}

.results-table tr th:first-child,
.results-table tr td:first-child {
    border-left: none;
}

.item-cell.item-cell {
    width: 128px;
}

td.item-cell.item-cell {
    background: lightgreen;
}

<table class="results-table">
  <thead>
    <tr>
      <th class="item-cell">
        Item
      </th>
      <th>
        Package
      </th>
      <th>
        Price
      </th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td class="item-cell">
        <a href="#">Link 1</a>
      </td>
      <td>Package 1</td>
      <td>Price 1</td>
    </tr>
    <tr>
      <td class="item-cell">
        <a href="#">Link 2</a>
      </td>
      <td>Package 2</td>
      <td>Price 2</td>
    </tr>
    <tr>
      <td class="item-cell">
        <a href="#">Link 3</a>
      </td>
      <td>Package 3</td>
      <td>Price 3</td>
    </tr>
  </tbody>
</table>

By the way, a shadow works in Firefox, but does not work in other browsers.

Firefox [image]

Chrome, Opera, Edge, IE11 [image]

So how to implement a shadow for the thead?

Thanks.

Solution

add z-index negative on td

.results-table td{
    position: relative;
    z-index: -1;
}