html参考手册,请问BODY中的ONLOAD是什么意思？

html参考手册,请问BODY中的ONLOAD是什么意思？详细介绍

/*浏览器默认设置*/
*{
padding: 0;margin: 0;font-size: 12px;font-family: Verdana,Arial,'宋体';
}
a{
text-decoration: none;color: black;
}
a:hover{
text-decoration: underline;
}
img{
border: 1px solid #e7e7e7;
}
li{
list-style-type: none;
}
/*设置显示主窗体*/
.wrap{
width: 1129px;margin: 0 auto;
border-left: 2px solid #e7e7e7;
border-right: 2px solid #e7e7e7;
text-align:left;
}
.content,.head,.footer{
margin-left: 12px;
margin-right: 12px;
}
/*设置顶部*/
.head{
text-align: center;
}
.header ul{
width: 1100px;
position: relative;
height: 50px;
}
.header ul li{
position:absolute;
height: 50px;
top:0;
}
.header ul li a{
font-size: 14px;
line-height: 50px;
}
.header ul li a:hover{
border-bottom: 3px solid #cc0000;
text-decoration: none;
}
#nav1{
left: 0;width: 155px;
}
#nav2{
left: 155px;width: 157px;
}
#nav3{
left: 312px;width: 167px;
}
#nav4{
left: 479px;width: 158px;
}
#nav5{
left: 637px;width: 101px;
}
#nav6{
left: 738px;width: 185px;
}
#nav7{
left: 923px;width: 177px;
}
/*设置正文左部*/
.contentleft{
float: left;
width: 180px;
background-color: #f9f9f9;
}
.contentleft h2{
font-size: 20px;
text-align: center;
background-color: #bebebe;
color: white;
font-weight: normal;
height: 50px;
line-height: 50px;
}
#optional{
margin-top: 10px;
}
.contentleft ul li{
padding-left: 5px;
}
.contentleft h3{
margin-top: 10px;
margin-bottom: 5px;
}
/*设置正文右半部分*/
.contentright{
float: left;
width: 720px;
margin: 15px 0 0 10px;
}
/*清除浮动*/
.content{
width: 100%;
overflow: hidden;
}
.htmlcourse{
font-size: 25px;
border-bottom: 1px solid #aaa;
}
/*设置侧边栏*/
.contentprofile{
float: left;
width: 180px;
margin-left: 10px;
overflow: hidden;
}
.chs a:hover{
color: red;
}
.ch,.chs{
border-bottom: 2px solid #aaa;
line-height: 30px;
}
.contentright h2{
line-height: 50px;
}
.names{
background-color: #f88e8b;
color: white;
width: 100px;
height: 30px;
margin-bottom: 10px;
}
.name{
margin-bottom: 10px;
margin-left: 20px;
}
/*设置页底*/
.footer{
background-color: #b6b6b6;
margin-top: 30px;
margin-bottom: 10px;
}