html {
    font-size: 16px;
}
@media (max-width: 768px) {
    html {
        font-size: 12px;
    }
}

body {
    font-family: Arial, sans-serif;
    margin: 0;
    padding: 0;
}

header {
    text-align: center;
    background-color: #333;
    color: #fff;
    padding: 1rem;
}

.nav_list {
    list-style-type: none;
    padding: 4px 0;
    margin: 0;
    display: flex;
    justify-content: center;
    align-items: center;
    background-color: #fff;
}

.nav_item a {
    margin: 0 1rem;
    color: #333;
    text-decoration: none;
}

.nav_item a:hover {
    text-decoration: underline;
}

main > div {
    border: 1px solid #333;
    border-radius: 4px;
    padding: 1rem;
    margin: 1rem;
}

h2 {
    background-color: #333;
    color: #fff;
    border-radius: 4px;
}

h2, h3 {
    margin: 0;
    padding: 0;
    text-align: center;
}

h3 {
    text-decoration: underline;
}

main p, main span {
    border: 1px solid #333;
    margin: 1rem;
    padding: 1rem;
}

.text_align_center {
    text-align: center;
}
.text_align > p, .text_align > span, .text_align > img, .text_align > div > p {
    text-align: center;
}

.margin_0_auto {
    width: 50%;
    margin: 0 auto;
}
#margin_0_auto_img {
    width: 200px;
    height: 300px;
}

.flex {
    display: flex;
    justify-content: center;
    align-items: center;
}

.grid {
    display: grid;
    place-items: center;
}

.relative {
    position: relative;
    width: 100%;
    height: 300px;
}
.absolute {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

footer {
    text-align: center;
    background-color: #333;
    color: #fff;
    padding: 1rem;
}

pre {
    margin: 1em 0; /* ブロック前後の余白 */
    padding: 1em; /* ブロック内の余白 */
    border-radius: 5px; /* 角丸 */
    background: #25292f; /* 背景色 */
    color: #fff; /* 文字色 */
    white-space: pre-wrap; /* はみ出たときに折り返す */
}