add(web.test4): 新增 web 课实验四

Web/test4/public/mainMenu.css

@@ -0,0 +1,59 @@
+* {
+  margin: 0;
+  padding: 0;
+  font-size: 24px;
+  box-sizing: border-box;
+}
+
+ul li {
+  list-style: none;
+}
+
+.sub-menu {
+  display: none;
+}
+
+ul > li:hover .sub-menu {
+  display: block;
+}
+
+a {
+  text-decoration: none;
+  color: inherit;
+}
+
+.main-menu li {
+  background: #c22fef;
+  color: aliceblue;
+  border-radius: 2px;
+  text-align: center;
+}
+
+.main-menu > li {
+  width: 200px;
+  border: 1px solid #77049a;
+  margin: 1px 3px;
+}
+
+li:hover {
+  background: #a93aca;
+}
+
+div {
+  border: 15px solid red;
+  border-bottom-color: transparent;
+  border-left-color: transparent;
+  border-right-color: transparent;
+  display: inline-block;
+  position: absolute;
+  right: 2px;
+  opacity: 0;
+}
+
+.main-menu li {
+  position: relative;
+}
+
+ul > li:hover div {
+  opacity: 1;
+}