.at-collapsible__block {
    max-height: 0;
    opacity: 0;
    transition: all .5s ease-in-out .2s;
}

.at-collapsible__heading--open + .at-collapsible__block {
    overflow: hidden;
    max-height: 1000px;
    opacity: 1;
}

.at-collapsible__heading {
    position: relative;
    padding-left: 20px; /** Same as .at-collapsible__block */
    cursor: pointer;
}

.at-collapsible__heading:before {
    position: absolute;
    left: 0;
    content: '+';
}

.at-collapsible__heading--open:before {
    content: '-';
    padding-left: .05em;
}

.at-collapsible__block {
    margin-left: 20px; /** Same as .at-collapsible__heading */
}