PC에서 모바일화면으로 전환되면 GNB 메뉴가 OPEN되어있는 상태 (소스)3

/* Google Web Font : Montserrat */
@import url('https://fonts.googleapis.com/css?family=Montserrat:200,300,400,500&display=swap');
@import url('https://fonts.googleapis.com/css?family=Manrope:300,400,500,600&display=swap');
@import url('https://fonts.googleapis.com/css2?family=Source+Sans+Pro:wght@200;300;400;600;700;900&display=swap');
@import url('https://fonts.googleapis.com/css2?family=Overpass&display=swap');

/* FontAwesome CDN 4.7 */
@import url('https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css');

/* source sans */
@import url('https://fonts.googleapis.com/css2?family=Source+Sans+3:wght@200;300;400;600&display=swap');

/* Overpass */
@import url('https://fonts.googleapis.com/css2?family=Overpass&display=swap');

/* Reset CSS */
* { box-sizing: border-box; }
ul { list-style: none; padding: 0; }
a { text-decoration: none; }

/* Default CSS */
body {
  font-family: 'Montserrat', sans-serif;
  color: #222;
  font-size: 15px;
  margin: 0;
  height: 100vh;
  background-color: #fff;
}

/* Entire Layout */
.cd-section {
  height: 100vh;
 }
.cd-section > div {
  height:100%;
  position: relative;
}
.content {
  background-color: #ddd;
  position: absolute;
  width: calc(100% - 40px);
  height: calc(100% - 80px);
  left: 20px;
  bottom: 20px;
  overflow: hidden;
}

/* Header */
header {
  position: fixed;
  top:0;
  left:0;
  width: 100%;
}
.gnb-inner {
  /* border: 1px solid #000; */
  width: calc(100% - 40px);
  margin: auto;
  height: 60px;
  line-height: 60px;
}
.logo {
  float: left;
}
.logo img {
  padding-top: 17px;
}
.gnb {
  float: right;
}
.menu {
  display: none;
}
.menu a {}
.slogan {
  font-size: 16px;
  font-style: italic;
}
.trigger {
  display: none;
}

/* Hiring button */
.btn-hiring {
  position: fixed;
  right: 50px;
  bottom: 50px;
  color: #fff;
  background-color: #000;
  padding: 10px 20px;
  border-radius: 20px;
  box-shadow: 5px 5px 20px rgb(0, 0, 0, 0.3);
  transition: 0.5s;
}
.btn-hiring .fa-solid {
  transform: rotateY(180deg);
  margin-right: 5px;
}
.btn-hiring:active {
  transform: scale(0);
}

/* ############# section : Home ############# */
.home-inner {
  background-color: #fff;
  height: 100%;
}
.home-inner::before {
  content: '';
  background-color: #000;
  position: absolute;
  width: 0;
  height: 100%;
  top: 0;
  left: 0;
  animation: overlay 1s 2.5s ease-in-out;
  /* display: none; */
}
.welcome-text {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  font-size: 50px;
  font-weight: 300;
  text-align: center;
  line-height: 1.2em;
  margin: 0;
  font-family: 'Source Sans 3', sans-serif;
  animation: slideup 2s linear;
  animation-fill-mode: both;
  /* display: none; */
}
.welcome-text span {
  display: block;
  font-size: 26px;
}
.welcome-text span .fa {
  color: crimson;
}
.opacity-image {
  background: url('images/section-bg-01.jpg') no-repeat center center;
  background-size: cover;
  height: 100%;
  opacity: 0;
  animation: opacity-image 1s 3s ease-in both;
  /* display: none; */
}
.home-heading {
  /* display: none; 
  border: 1px solid red;*/
  position: absolute;
  top: 50%;
  /* left: 40%; */
  left: 300px; 
  /* margin-left: 200px; */
  transform: translateY(-50%);
}
.home-heading span {
  display: block;
  font-size: 115px;
  color: #fff;
  font-weight: 600;
  width: 0;
  overflow: hidden;
  animation: reveal 1s 3s ease-in-out both;
}
.home-heading span:nth-child(1) {
  animation-delay: 3s;
}
.home-heading span:nth-child(2) {
  animation-delay: 3.2s;
}
.home-heading span:nth-child(3) {
  animation-delay: 3.4s;
}

@keyframes slideup {
  0% {
    opacity: 0;
    margin-top: 50px;
  }
  20% {
    opacity: 1;
    margin-top: 0;
  }
  80% {
    opacity: 1;
    margin-top: 0;
  }
  100% {
    opacity: 0;
  }
}
@keyframes overlay {
  0% {
    width: 0;
    left: 0;
  }
  50% {
    width: 100%;
    left: 0;
  }
  100% {
    width: 0;
    /* right: 0; */
    left: 100%;
  }
}
@keyframes opacity-image {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 1;
  }
}
@keyframes reveal {
  0% {
    width: 0;
  }
  100% {
    width: 100%;
  }
}