table タグの中身が空のセルに斜線（対角線）を入れる。
td:empty { background-image: linear-gradient( to bottom right, transparent 0%, transparent calc(50% - 1px), #000 calc(50% - 1px), #000 calc(50% + 1px), transparent calc(50% + 1px), transparent 100% ); }
×印（対角線2本）
background-image: linear-gradient(to bottom right, transparent 0%, transparent calc(50% - 1px), #000 calc(50% - 1px), #000 calc(50% + 1px), transparent calc(50% + 1px), transparent 100% ), linear-gradient(to bottom left, transparent 0%, transparent calc(50% - 1px), #000 calc(50% - 1px), #000 calc(50% + 1px), transparent calc(50% + 1px), transparent 100% );