jquery的选择器功能
1 :lt(index) selector
一组元素选择index之前的元素，若index<0 则倒着选过来
http://api.jquery.com/lt-selector/
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>lt demo</title>
<script src="//code.jquery.com/jquery-1.10.2.js"></script>
</head>
<body> <table border="1">
<tr><td>TD #0</td><td>TD #1</td><td>TD #2</td></tr>
<tr><td>TD #3</td><td>TD #4</td><td>TD #5</td></tr>
<tr><td>TD #6</td><td>TD #7</td><td>TD #8</td></tr>
</table> <script>
$( "td:lt(4)" ).css( "backgroundColor", "yellow" );
$( "td:lt(-2)" ).css( "color", "red" );
</script> </body>
</html>