border-collapse

значения свойства:

css/style.css

table, td{
 border: 1px solid #00F;
 font-size: 26px;
}

#t1 {
 border-collapse: collapse;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<table id="t1">
<tr>
<td>Hello World!</td>
<td>To be or not to be?</td>
</tr>
<tr>
<td>Привет Мир!</td>
<td>Быть или не быть?</td>
</tr>
</table>
<br />
<br />
<table id="t2">
<tr>
<td>Hello World!</td>
<td>To be or not to be?</td>
</tr>
<tr>
<td>Привет Мир!</td>
<td>Быть или не быть?</td>
</tr>
</table>
<br />
<br />
<table id="t3">
<tr>
<td>Hello World!</td>
<td>To be or not to be?</td>
</tr>
<tr>
<td>Привет Мир!</td>
<td>Быть или не быть?</td>
</tr>
</table>
<br />
<script>
/*JavaScript*/
document.getElementById("t2").style.borderCollapse="collapse";
/*jQuery*/
$(document).ready(function () {
$("#t3").css({ "border-collapse": "separate" });
});
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<table id="t1">
<tr>
<td>Hello World!</td>
<td>To be or not to be?</td>
</tr>
<tr>
<td>Привет Мир!</td>
<td>Быть или не быть?</td>
</tr>
</table>
<br />
<br />
<table id="t2">
<tr>
<td>Hello World!</td>
<td>To be or not to be?</td>
</tr>
<tr>
<td>Привет Мир!</td>
<td>Быть или не быть?</td>
</tr>
</table>
<br />
<br />
<table id="t3">
<tr>
<td>Hello World!</td>
<td>To be or not to be?</td>
</tr>
<tr>
<td>Привет Мир!</td>
<td>Быть или не быть?</td>
</tr>
</table>
<br />
<script>
/*JavaScript*/
document.getElementById("t2").style.borderCollapse="collapse";
/*jQuery*/
$(document).ready(function () {
$("#t3").css({ "border-collapse": "separate" });
});
</script>
</body>
</html>