Alban,
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<html>
<head>
<meta charset="utf-8">
<script>
var day = (new Date).getDay()||7;
document.write(
'<style>',
'td:nth-of-type('+day+') { background-color: red;}',
'</style>'
)
</script>
</head>
<body>
<table cols="7">
<tr bgcolor="#EEf0F5" align="center">
<td id="PN">ПН</td>
<td id="VT">ВТ</td>
<td id="SR">CР</td>
<td id="CH">ЧТ</td>
<td id="PT">ПТ</td>
<td id="SB">СБ</td>
<td id="VS">ВС</td>
</tr>
<tr align="center">
<td id="PN">9<sup>00</sup></td>
<td id="VT">9<sup>00</sup></td>
<td id="SR">9<sup>00</sup></td>
<td id="CH">9<sup>00</sup></td>
<td id="PT">9<sup>00</sup></td>
<td id="SB"></td>
<td id="VS"></td>
</tr>
<tr align="center">
<td id="PN">18<sup>00</sup></td>
<td id="VT">18<sup>00</sup></td>
<td id="SR">18<sup>00</sup></td>
<td id="CH">18<sup>00</sup></td>
<td id="PT">18<sup>00</sup></td>
<td id="SB"></td>
<td id="VS"></td>
</tr>
</table>
</body>
</html>