add backspace button

index.html

@@ -45,6 +45,8 @@
 					<button id="3" type="button">3</button>
 
 					<button id="0" type="button">0</button>
+
+					<button id="backspace" type="button">⌫</button>
 				</div>
 
 				<div id="operators">

styles.css

@@ -60,6 +60,10 @@ label[for="expr"] {
 	grid-template-columns: subgrid;
 }
 
+#backspace {
+	grid-column: 3;
+}
+
 #operators {
 	grid-column: 4;
 	grid-row: 1 / 3;