*{
    margin: 0;
    padding:0;
}
canvas{
    background-color: #ccc;
}
.tip{

}

.return-btn{
    display: none;
    clear: both; padding: 10px 0;
}
.return-btn a{
    text-align: center;width: 100px; height: 30px; display: block; background-color: #ccc; line-height: 30px;
}

pre{
    background-color: #ccc;
    max-width: 100%;
    overflow-x: auto;
}

@media screen and (max-width: 640px) {
    .return-btn{
        display: block;
    }
}