feat: the flex property

Author: Uribouz

starter/04-CSS-Layouts/flexbox.html

@@ -50,9 +50,29 @@
         margin-right: 30px;
       } */
 
+      .el {
+        /* DEFAULTS:
+        flex-grow: 0;
+        flex-shrink: 1;
+        flex-basis: auto; */
+
+        /* * Not make the child elements shrink
+        flex-basis: 400px;
+        flex-shrink: 0; 
+        =
+        flex: 0 0 400px;
+        */
+
+        flex-basis: 100px;
+        /* flex-grow: 1; */
+        flex: 1;
+      }
+
       .el--1 {
         /* Make this element vertically at the top of the container */
         align-self: flex-start;
+
+        flex-grow: 2;
       }
 
       .el--5 {