Simple Fade In Effect

index.html

 <div class="container-about-us">    
      <ul class="white about-us">
        <li><a href="#" class="text-a-about">Web Design</a></li>
        <li><a href="#" class="text-a-about">Marketing</a></li>
        <li><a href="#" class="text-a-about">SEO</a></li>
      </ul>
  </div>

style.css

/* Containers */
.container-about-us {
  height: 50vh;
}

/* Elements */
/* About Us Container */
.about-us {
  position: absolute;
  top: 30%;
  left: 50%;
  transform: translate(-50%, -70%);
  list-style: none;
}
.text-a-about {
  color: white !important;
  text-decoration: none;
  font-weight: 900;
  background: #232323;
  padding: 15px 25px;
  border-radius: 25px;
  transition: 0.3s ease;
}
.text-a-about:hover {
  background: var(--light-blue);
}
.about-us li {
  display: inline-block;
  white-space: nowrap;
}
.about-us li:not(:last-child) {
  margin-right: 120px;
}
/* Web Design */
.about-us li:first-child:hover::before {
  content: "";
  z-index: -1;
  width: auto;
  height: 300px;
  display: block;
  position: absolute;
  top: 30px;
  left: 0;
  right: 0;
  margin: auto;
  background: url(http://192.168.1.122/dev-tlg/wp-content/uploads/2018/11/web-design.png)
    50% 50%;
  background-size: contain;
  background-repeat: no-repeat;
  animation: fade-in 2s ease-in-out;
}
/* Marketing */
.about-us li:nth-child(2):hover::before {
  content: "";
  z-index: -1;
  width: auto;
  height: 300px;
  display: block;
  position: absolute;
  top: 30px;
  left: 0;
  right: 0;
  margin: auto;
  background: url(http://192.168.1.122/dev-tlg/wp-content/uploads/2018/11/marketing.png)
    50% 50%;
  background-size: contain;
  background-repeat: no-repeat;
  animation: fade-in 2s ease-in-out;
}
/* SEO */
.about-us li:nth-child(3):hover::before {
  content: "";
  z-index: -1;
  width: auto;
  height: 300px;
  display: block;
  position: absolute;
  top: 30px;
  left: 0;
  right: 0;
  margin: auto;
  background: url(http://192.168.1.122/dev-tlg/wp-content/uploads/2018/11/seo.png)
    50% 50%;
  background-size: contain;
  background-repeat: no-repeat;
  animation: fade-in 2s ease-in-out;
}

@keyframes fade-in {
  0% {
    opacity: 0;
    top: 0px;
  }
  50% {
    opacity: 1;
    top: 30px;
  }
  100% {
    opacity: 1;
    top: 30px;
  }
}