ios问题，如何将界面设置成不规则的九宫格形状

var a = Math.ceil(Math.random()*30)+10;
var b = Math.ceil(Math.random()*30)+10;
var c = Math.ceil(Math.random()*30)+10;
var d = Math.ceil(Math.random()*30)+10;
document.write('<table width="500" height="900" border="1"><tr>');
document.write('<td width="'+a+'%" height="'+b+'%"></td>');
document.write('<td width="'+c+'%"></td>');
document.write('<td></td>');
document.write('</tr><tr>');
document.write('<td height="'+d+'%"></td>');
document.write('<td></td>');
document.write('<td></td>');
document.write('</tr><tr>');
document.write('<td></td>');
document.write('<td></td>');
document.write('<td></td>');
document.write('</tr></table>');