solution.css

:root {
    --color-black: #272727;
    --size: 40px;
}

body {
    color: var(--color-black);
    font: 16px/1em arialregular;
    margin: 0;
    padding: 0;
}

.crossword {
    background: var(--color-black);
    border: 1px solid var(--color-black);
    display: block;
    box-sizing: border-box;
}

.crossword::after {clear: both;}
.crossword.w-1, .space.w-1 {width: calc(var(--size) * 1);}
.crossword.w-2, .space.w-2 {width: calc(var(--size) * 2);}
.crossword.w-3, .space.w-3 {width: calc(var(--size) * 3);}
.crossword.w-4, .space.w-4 {width: calc(var(--size) * 4);}
.crossword.w-5, .space.w-5 {width: calc(var(--size) * 5);}
.crossword.w-6, .space.w-6 {width: calc(var(--size) * 6);}
.crossword.w-7, .space.w-7 {width: calc(var(--size) * 7);}
.crossword.w-8, .space.w-8 {width: calc(var(--size) * 8);}

.letter {
    background: white;
    border: 1px solid var(--color-black);
    float: left;
    display: block;
    box-sizing: border-box;
    width: var(--size);
    height: var(--size);
    line-height: var(--size);
    text-align: center;
}

.letter::after {
    content: attr(number);
    line-height: 1.3;
    font-size: 75%;
    position: absolute;
    left: calc(var(--size) / 12);
    top: 0;
}

.space {
    float: left;
    display: block;
    height: var(--size);
}