button{
outline: 1px solid transparent;
outline-offset: 5px;
transition: all 500ms;
}
button:focus{
outline: 1px solid rgba(0, 0, 0, 0.5);
outline-offset: 0;
}
outline-offsetプロパティでoutlineとボタンの距離（オフセット）を
指定・変更することで残像のような効果を表しています。
※IE非対応
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