style.css

body {
    font-family: Arial,sans-serif;
}

fieldset {
    padding-left: 0;
    border: none;
}
input {
    padding-top: 0.25rem;
    padding-bottom: 0.25rem;
    padding-left: 0.25rem;
    padding-right: 0.25rem;
    border-radius: 3px;
    border: 1px solid #ccc;
    min-width: 240px;
}
input.error {
    border: 1px solid red;
}

.error-message {
    display: none;
    padding-left: 84px;
    font-size: 12px;
    font-style: italic;
}

input.error + .error-message {
    display: block;
}

label {
    display: inline-block;
    min-width: 80px;
}

button[type="submit"] {
    padding-top: .5rem;
    padding-bottom: .5rem;
    padding-left: 1rem;
    padding-right: 1rem;
    border: 1px solid #ccc;
    border-radius: 3px;
    font-size: 13px;
    outline: none;
    background: 0 0;
    user-select: none;
    white-space: nowrap;
    text-decoration: none;
    cursor: pointer;
}

button[type="submit"]:hover {
    border-color: #777;
}

button[type="submit"]:disabled {
    opacity: .3;
}

button[type="submit"]:disabled:hover {
    border: 1px solid #ccc;
}

#resultContainer {
    display: none;
    position: relative;
    margin-top: 2rem;
    padding: 1rem;
    border-radius: 3px;
    border: 1px solid #ccc;
}

#resultContainer::before {
    content: 'Server response:';
    position: absolute;
    top: 0;
    left: 0;
    top: -12px;
    left: 20px;
    padding: .25rem .5rem;
    border-radius: 3px;
    background: gainsboro;
    font-size: 13px;
    font-weight: bold;
}

#resultContainer.success,
#resultContainer.error,
#resultContainer.progress {
    display: block;
}

#resultContainer.success {
    border-color: #070;
    color: #070;
}
#resultContainer.success::before {
    background: #070;
    color: #fff;
}

#resultContainer.error {
    border-color: #d00;
    color: #d00;
}
#resultContainer.error::before {
    background: #d00;
    color: #fff;
}

#resultContainer.progress {
    border-color: #669;
    border-style: dashed;
    color: #669;
}
#resultContainer.progress::before {
    background: #669;
    color: #fff;
}