* {
    font-family: sans;
}

h1 {
    text-align: center;
}

div {
    display: flex;
    column-gap: 0.5em;
    row-gap: 0.5em;
    margin: 1em auto;
    justify-content: center;
    text-align: center;
}

#grid {
    border-spacing: 0;
    border-radius: 6px;
}

td {
    padding: 0;
}

#grid tr:first-child td:first-child 
{
    border-top-left-radius: 6px;
}

#grid tr:first-child td
{
    border-top: 2px solid black;
}

#grid tr:first-child td:last-child 
{
    border-top-right-radius: 6px;
}

#grid tr td:first-child 
{
    border-left: 2px solid black;
}

#grid tr td:last-child 
{
    border-right: 2px solid black;
}

#grid tr:last-child td:first-child 
{
    border-bottom-left-radius: 6px;
}

#grid tr:last-child td
{
    border-bottom: 2px solid black;
}

#grid tr:last-child td:last-child 
{
    border-bottom-right-radius: 6px;
}

.regionTop {
    border-top: 1px solid black;
}

.regionMiddle {
    border-top: 1px solid grey;
    border-bottom: 1px solid grey;
}

.regionBottom {
    border-bottom: 1px solid black;
}

.regionLeft {
    border-left: 1px solid black;
}

.regionCenter {
    border-left: 1px solid grey;
    border-right: 1px solid grey;
}

.regionRight {
    border-right: 1px solid black;
}

input {
    width: 1.6em;
    height: 1.6em;
    font-size: 1.5em;
    border: 0;
    padding: 0;
    text-align: center;
    transition: 0.3s;
}

input:read-write {
    color: darkblue;
    background: white;
}

input:read-only {
    color: black;
    background: #6666ff;
    font-weight: bold;
}

input.same-value:read-write {
    color: #009973;
    background: #66ffd9;
}

input.forbidden-value:read-write {
    background: #ccffe6;
}

input.same-value:read-only {
    color: #00664d;
    background: #00e6ac;
}

input::placeholder {
    color: #888;
}

#buttons {
    column-gap: 0.2em;
    margin: 0;
}

a {
    text-decoration: none;
}