鼠标移上去样式修改求助


<table cellspacing="0" cellpadding="4" rules="all" border="1" id="MainContent_GridView1" style="color:#333333;border-collapse:collapse;">
<tr class="ui-state-default" style="height:35px;">
<th scope="col">
                    <input id="MainContent_GridView1_all2" type="checkbox" name="ctl00$MainContent$GridView1$ctl01$all2" />
                </th><th scope="col"><a href="javascript:__doPostBack(&#39;ctl00$MainContent$GridView1&#39;,&#39;Sort$Code&#39;)" style="color:#333333;">Code</a></th><th scope="col"><a href="javascript:__doPostBack(&#39;ctl00$MainContent$GridView1&#39;,&#39;Sort$Name&#39;)" style="color:#333333;">Name</a></th><th scope="col"><a href="javascript:__doPostBack(&#39;ctl00$MainContent$GridView1&#39;,&#39;Sort$Url&#39;)" style="color:#333333;">Url</a></th><th scope="col"><a href="javascript:__doPostBack(&#39;ctl00$MainContent$GridView1&#39;,&#39;Sort$PCode&#39;)" style="color:#333333;">PCode</a></th>
</tr><tr onmouseover="yy_sgv_changeMouseOverCssClass(this, &#39;OverRow&#39;)" onmouseout="yy_sgv_changeMouseOverCssClass(this)" style="background-color:#E2E4FF;">
<td>
                    <input id="MainContent_GridView1_item2_0" type="checkbox" name="ctl00$MainContent$GridView1$ctl02$item2" />
                </td><td align="center" style="width:200px;">1</td><td align="center" style="width:200px;">项   1</td><td align="center" style="width:400px;">Http://11111111111</td><td align="center" style="width:200px;">1</td>
</tr><tr onmouseover="yy_sgv_changeMouseOverCssClass(this, &#39;OverRow&#39;)" onmouseout="yy_sgv_changeMouseOverCssClass(this)" style="background-color:White;">
<td>
                    <input id="MainContent_GridView1_item2_1" type="checkbox" name="ctl00$MainContent$GridView1$ctl03$item2" />
                </td><td align="center" style="width:200px;">2</td><td align="center" style="width:200px;">项   2</td><td align="center" style="width:400px;">Http://11111111111</td><td align="center" style="width:200px;">2</td>
</tr><tr onmouseover="yy_sgv_changeMouseOverCssClass(this, &#39;OverRow&#39;)" onmouseout="yy_sgv_changeMouseOverCssClass(this)" style="background-color:#E2E4FF;">
</td></tr></table>
onmouseover只是个样式赋值this.className="样式名称"
为什么有后面的style="background-color:#E2E4FF;"我就没效果，没有我就有效果了？？
(这个初始样式必须有的)