.rendu{font-family:'Georgia'; 
       font-style:italic; 
       font-size:x-large;
    }
.rendu:empty:before{content:"Editez une expression!"}

.rendu:empty input[type=button]{display:none}

div.bloc{display:inline-block;}
div.bloc:before{content:'('}
div.bloc:after{content:')'}
div.frac{display:inline-block; 
         vertical-align:middle;
         text-align:center;
         padding:4px;
        }
div.frac>div:first-child{border-bottom:1px solid blue;} 
div.frac div.frac{font-size:smaller}
.ope{padding:4px;}