Create Tic-Tac-Toe in HTML/CSS. Understand layers (z-index).

tictactoe.css

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+body {
+  text-align: center;
+  margin: 0;
+}
+
+.tic-tac-toe {
+  height: 450px;  
+  width: 450px;  
+  margin: 50px auto 30px auto;
+  position: relative;
+  background-color: #E6E6FA;
+}
+
+.tic-tac-toe input[type="radio"] {
+  display: none;
+}
+
+.tic-tac-toe input[type="radio"].turn-1 + label {
+  z-index: 1;
+  display: block;
+}
+
+.tic-tac-toe input[type="radio"].turn-1:checked + label {
+  cursor: default;
+  background-color: red;
+  z-index: 10 !important;
+}
+
+.tic-tac-toe input[type="radio"].turn-1.field-9:checked ~ .turn-2.field-3 + label {
+  display: block;
+  cursor: default;
+  background-color: green;
+  z-index: 2;
+}
+
+.tic-tac-toe label {
+  background-color: #78bec5;
+  height: 140px;
+  width: 140px; 
+  display: none;
+  margin: 5px;
+  position: absolute;
+  cursor: pointer;
+  color: #fff;
+  -moz-transition: background-color 0.3s;
+  -o-transition: background-color 0.3s;
+  -webkit-transition: background-color 0.3s;
+  transition: background-color 0.3s;
+}
+
+.tic-tac-toe input[type="radio"].left + label {
+  left: 0px;
+  right: 300px;
+}
+
+.tic-tac-toe input[type="radio"].middle + label {
+  left: 150px;
+  right: 150px;
+}
+
+.tic-tac-toe input[type="radio"].right + label {
+  left: 300px;
+  right: 0px;
+}
+
+
+.tic-tac-toe input[type="radio"].top + label {
+  top: 0px;
+  bottom: 300px;
+}
+
+.tic-tac-toe input[type="radio"].center + label {
+  top: 150px;
+  bottom: 150px;
+}
+
+.tic-tac-toe input[type="radio"].bottom + label {
+  top: 300px;
+  bottom: 0px;
+}
+