一款纯css3如何实现的动画加载导航

<ul class="main-menu">
<li class="main-menu-item active"><a rel="nofollow noopener noreferrer" href="#">Home</a></li><li class="main-menu-item">
<a rel="nofollow noopener noreferrer" href="#">About Us</a></li><li class="main-menu-item"><a rel="nofollow noopener noreferrer" href="#">Another Link</a></li><li
class="main-menu-item"><a rel="nofollow noopener noreferrer" href="#">And another</a></li><li class="main-menu-item"><a
rel="nofollow noopener noreferrer" href="#">Stuff</a></li><li class="main-menu-item"><a rel="nofollow noopener noreferrer" href="#">Help</a></li><li class="main-menu-item">
<a rel="nofollow noopener noreferrer" href="#">Contact</a></li></ul>

*, *:after, *:before {
-moz-box-sizing: border-box;
box-sizing: border-box;
margin: 0;
padding: 0;
}
html, body {
background: #f0f0f0;
font-family: Helvetica, sans-serif;
height: 100%;
}
.main-menu {
margin: auto;
width: 960px;
text-align: center;
position: relative;
list-style-type: none;
margin-top: 2em;
}
.main-menu::after,
.main-menu-item.active::after {
content: '';
display: inline-block;
position: absolute;
}
.main-menu::after {
width: 70%;
height: 2px;
background: #9B1C27;
-webkit-animation: menuLine 1s ease forwards;
animation: menuLine 1s ease forwards;
-webkit-transform: scaleX(0);
-ms-transform: scaleX(0);
transform: scaleX(0);
bottom: 0;
left: 0;
right: 0;
margin: auto;
opacity: 0;
}
@-webkit-keyframes menuLine {
to {
-webkit-transform: scaleX(1);
transform: scaleX(1);
opacity: 1;
}
}
@keyframes menuLine {
to {
-webkit-transform: scaleX(1);
transform: scaleX(1);
opacity: 1;
}
}
@-webkit-keyframes menuItem {
to {
-webkit-transform: translateY(0);
transform: translateY(0);
opacity: 1;
}
}
@keyframes menuItem {
to {
-webkit-transform: translateY(0);
transform: translateY(0);
opacity: 1;
}
}
.main-menu-item a {
display: block;
padding: 1em;
text-decoration: none;
color: #555;
opacity: 0;
-webkit-transform: translateY(40%);
-ms-transform: translateY(40%);
transform: translateY(40%);
-webkit-animation: menuItem .8s ease forwards;
animation: menuItem .8s ease forwards;
-webkit-transition: all .2s ease;
transition: all .2s ease;
}
.main-menu-item:nth-child(4) a {
-webkit-animation-delay: .4s;
animation-delay: .4s;
}
.main-menu-item:nth-child(3) a, .main-menu-item:nth-child(5) a {
-webkit-animation-delay: .5s;
animation-delay: .5s;
}
.main-menu-item:nth-child(2) a, .main-menu-item:nth-child(6) a {
-webkit-animation-delay: .6s;
animation-delay: .6s;
}
.main-menu-item:nth-child(1) a, .main-menu-item:nth-child(7) a {
-webkit-animation-delay: .7s;
animation-delay: .7s;
}
.main-menu-item {
display: inline-block;
position: relative;
}
.main-menu-item:hover a, .main-menu-item.active {
color: #9B1C27;
}
.main-menu-item:hover a::after, .main-menu-item.active::after {
width: 0;
height: 0;
border-bottom: .5em solid #9B1C27;
border-left: .5em solid transparent;
border-right: .5em solid transparent;
bottom: 0;
left: 0;
right: 0;
margin: auto;
opacity: 0;
-webkit-transform: translateY(0.05em);
-ms-transform: translateY(0.05em);
transform: translateY(0.05em);
-webkit-animation: menuItem .8s 1.2s ease forwards;
animation: menuItem .8s 1.2s ease forwards;
}