#demoIzq, #demoDer { border:1px dashed; width:306px; background-color:#EAEAEA; text-align:left; margin:0 1px 0 1px; }
#demoMed { border:1px dashed; width:306px; background-color:#EAEAEA; text-align:center; margin:0 1px 0 1px; }

#demoDer, #demoMed { float:left; }

select { width:300px; font-size:12px; }