JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>table test</title>
</head>
<body>
  <table>
  <tr>
    <th class="t"></th>
    <th>TOP - Right label</th>
    <th class="te"></th>
    <th rowspan=2>some label</th>
  </tr>
  <tr>
    <th class="bs"></th>
    <th>BOTTOM label</th>
    <th class="b"></th>
  </tr>
  <tr>
    <td colspan=3>some text</td>
    <td>some text</td>
  </tr>
</table>
</body>
</html>

CSS

table {
  border: 2px solid red;
  border-collapse: collapse;
  box-sizing: border-box;
}
table th, table td {
  background-color: #eff;
  border: 1px solid gray;
  padding: 0.2em 0.5em; 
}

table th {
  background-color: #a85;
  color: #eee;
  border: 1px solid #5f5;
}

.t {
  position: relative;
  border-bottom: none;
  border-right: none;
}

.t + * {
  border-left: none;
  border-right: none;
}

.bs {
  border-top: none;
  border-right: none;
}

.bs + * {
  border-left: none;
  border-right: none;
}

.t::before, .b::before {
  content: "";
  height: 100%;
  background-color: none;
  border-left: 1px solid #5f5;
  transform: rotate(-45deg);
  transform-origin: bottom;
  position: absolute;
  left: 100%;
  top: 0;
}

.b::before {
  transform-origin: top;
  left: -1px;
}

.b {
  position: relative;
  border-left: none;
  border-top: none;
}

.te {
  border-left: 0;
  border-bottom: 0;
}