アコーディオン作成・CSSのみ

<input id="acd-check1" class="acd-check" type="checkbox">
<label class="acd-label" for="acd-check1">クリックで開く1</label>
<div class="acd-content">
    <p>hello.world!</p>
</div>

<input id="acd-check2" class="acd-check" type="checkbox">
<label class="acd-label" for="acd-check2">クリックで開く2</label>
<div class="acd-content">
    <p>hello.world2!</p>
</div>
以下同じ

.acd-check{
    display: none;
}
.acd-label{
    background: #333;
    color: #fff;
    display: block;
    margin-bottom: 1px;
    padding: 10px;
}
.acd-content{
    border: 1px solid #333;
    height: 0;
    opacity: 0;
    padding: 0 10px;
    transition: .5s;
    visibility: hidden;
}
.acd-check:checked + .acd-label + .acd-content{
    height: 40px;
    opacity: 1;
    padding: 10px;
    visibility: visible;
}

.acd-check{
    display: none;
}
.acd-label{
    background: #333;
    color: #fff;
    display: block;
    margin-bottom: 1px;
    padding: 10px;
}
.acd-label:after{
    background: #00479d;
    box-sizing: border-box;
    content: '\f067';
    display: block;
    font-family: "Font Awesome 5 Free";
    height: 52px;
    padding: 10px 20px;
    position: absolute;
    right: 0;
    top: 0px;
}
.acd-content{
    border: 1px solid #333;
    height: 0;
    opacity: 0;
    padding: 0 10px;
    transition: .5s;
    visibility: hidden;
}
.acd-check:checked + .acd-label:after{
    content: '\f068';
}
.acd-check:checked + .acd-label + .acd-content{
    height: 40px;
    opacity: 1;
    padding: 10px;
    visibility: visible;
}