根据给定的元素名匹配元素

node2:/var/www/html/jquery#cat t18.html 
<li id='aaa' class='mmmm'>1111111111</id>
<li id='test' class='nnnn'>222222222</id>
<p>aaaaaaaaaaaaa</p>
<li id='ccc' class='kkkkk'>3333333333</id>
<li id='ddd' class='hhhh'>44444444444</id>
<p>bbbbbbbbbbbbb</p>
<li id='fff' class='kkkkk'>555555555</id>
<script type="text/javascript" src="jquery-2.2.2.min.js"></script>
<script type="text/javascript" src="t18.js"></script>

      
node2:/var/www/html/jquery#cat t18.js
$("p").css("background","red");