<form name="calculator">
  <table>
    <tr>
      <td colspan="4">
        <input type="text" name="display" disabled>
      </td>
    </tr>
    <tr>
      <td><input type="button" name="one" value="1" onclick="calculator.display.value += 1"></td>
      <td><input type="button" name="two" value="2" onclick="calculator.display.value += 2"></td>
      <td><input type="button" name="three" value="3" onclick="calculator.display.value += 3"></td>
      <td><input type="button" class="operator" name="plus" value="+" onclick="calculator.display.value += '+'"></td>
    </tr>
    <tr>
      <td><input type="button" name="four" value="4" onclick="calculator.display.value += 4"></td>
      <td><input type="button" name="five" value="5" onclick="calculator.display.value += 5"></td>
      <td><input type="button" name="six" value="6" onclick="calculator.display.value += 6"></td>
      <td><input type="button" class="operator" name="minus" value="-" onclick="calculator.display.value += '-'"></td>
    </tr>
    <tr>
      <td><input type="button" name="seven" value="7" onclick="calculator.display.value += 7"></td>
      <td><input type="button" name="eight" value="8" onclick="calculator.display.value += 8"></td>
      <td><input type="button" name="nine" value="9" onclick="calculator.display.value += 9"></td>
      <td><input type="button" class="operator" name="times" value="x" onclick="calculator.display.value += '*'"></td>
    </tr>
    <tr>
      <td><input type="button" class="operator" id="clear" name="clear" value="C" onclick="calculator.display.value = ''"></td>
      <td><input type="button" class="operator" name="zero" value="0" onclick="calculator.display.value += '0'"></td>
      <td><input type="button" class="operator" name="doit" value="=" onclick="calculator.display.value = eval(calculator.display.value)"></td>
      <td><input type="button" class="operator" name="div" value="/" onclick="calculator.display.value += '/'"></td>
    </tr>
  </table>
</form>
form {
  background: #ccc;
  margin: 0 auto;
  width: 300px;
  padding: 20px;
  border-radius: 5px;
}
form * {
  box-sizing: border-box;
}
form table {
  width: 100%;
  border-collapse: collapse;
}
form table tr, form table td {
  border: 1px solid #eee;
}
form table tr:first-child input {
  height: 50px;
  padding: 0 10px;
  font-weight: 700;
}
form table tr:not(:first-child) td {
  height: 70px;
}
form table tr:not(:first-child) td input {
  height: 100%;
}
form table input {
  width: 100%;
  display: block;
  border: 0;
  font-size: 16px;
}
form table input:focus {
  outline: none;
}
form table input:hover {
  background: #eee;
  transition: all .2s ease-in-out;
}
form table input.operator {
  font-weight: 700;
  color: #fff;
  background: #ffcc00;
}

External CSS

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