<table>
  <thead>
    <tr><th>1</th><th>2</th></tr>
  </thead>
  <tr><td>3</td><td>4</td></tr>
  <tr><td>5</td><td>6</td></tr>
  <tr><td>7</td><td>8</td></tr>
</table>
tr:nth-child(odd) td { background-color: red; } /* нечетная */
tr:nth-child(even) td { background-color: blue; } /* четная */

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