I can use css to draw a semicircle like:
background: none repeat scroll 0 0 #CB1D1E;
border-radius: 0px 0px 70px 70px;
margin: 15px 0 0 20px;
and of course I am able to add a link for it to make it be clickable.
<a href="1111.html"> but I just want only the semicircle be clickable, but not a rectangle! I found some white space around the semicircle could be also clickable, which is not what I want!...