.rectangle {
    width: 370px;
    margin: 50px auto;
    font-size: 1.1em;
}

input, label, span {
    margin: 5px 0;
    font-size: 1em;
    padding-left: 5px;
}

input:invalid+span:after {
    content: '✖';
    color: red;
}

input:valid+span:after {
    content: '✓';
    color: green;
}

#rectangle-calc {
    width: 70px;
    margin-left: 63px;
}

#rectangle-width-validate,
#rectangle-height-validate {
    color: red;
    margin-left: 63px;
    font-size: 0.8em;
}