小练习
#aa{
width: 0px;
height:0px;
border-top: 100px solid blue;
border-left: 100px solid transparent;
border-right: 100px solid transparent;
border-bottom: 100px solid transparent;
}
.bb{
width: 100px;
height: 100px;
border: 3px solid blue;
border-top: 3px solid transparent;
border-right: 3px solid transparent;
transform: rotate(45deg);
margin-left: 100px;
}