style.css

@import url("https://fonts.googleapis.com/css?family=Muli&display=swap");
/* Importing a font from Google Fonts called Muli */

* {
  box-sizing: border-box;
}
/* Applying box-sizing: border-box to all elements in the document */

body {
  font-family: "Muli", sans-serif;
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
  overflow: hidden;
  margin: 0;
}

/* Styling the body element:
   - Using the font-family Muli with fallback to sans-serif
   - Displaying flexbox layout with centered alignment and justification
   - Setting the height of the body to occupy the full viewport height
   - Hiding any overflow beyond the body
   - Removing margin */

.container {
  display: flex;
  width: 90vw;
}
/* Styling the container class:
   - Displaying flexbox layout
   - Setting the width to 90% of the viewport width */

.panel {
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;
  height: 80vh;
  border-radius: 50px;
  color: #fff;
  cursor: pointer;
  flex: 0.5;
  margin: 10px;
  position: relative;
  -webkit-transition: all 700ms ease-in;
}
/* Styling the panel class:
   - Setting the background size to cover the element
   - Centering the background image
   - Preventing the background image from repeating
   - Setting the height to 80% of the viewport height
   - Applying a border radius of 50 pixels
   - Setting the text color to white
   - Changing the cursor to a pointer on hover
   - Setting the flex property to 0.5 of the available space
   - Setting a margin of 10 pixels around the panel
   - Creating a new stacking context for child absolutely positioned elements
   - Applying a CSS transition with a duration of 700ms and easing */

.panel h3 {
  font-size: 24px;
  position: absolute;
  bottom: 20px;
  left: 20px;
  margin: 0;
  opacity: 0;
}
/* Styling the h3 element inside panels:
   - Setting the font size to 24 pixels
   - Positioning it absolutely at the bottom-left corner of the panel
   - Removing any margins around the element
   - Initially setting the opacity to 0 to hide it */

.panel.active {
  flex: 5;
}
/* Styling the active panels:
   - Expanding the flex property to occupy 5 times the available space */

.panel.active h3 {
  opacity: 1;
  transition: opacity 0.3s ease-in 0.4s;
}
/* Styling the h3 element inside active panels:
   - Setting the opacity to 1 to make it visible
   - Applying a CSS transition to gradually change the opacity over 0.3 seconds with an initial delay of 0.4 seconds */

@media (max-width: 480px) {
  .container {
    width: 100vw;
  }
  /* Applying responsive styles when the viewport width is 480 pixels or less:
     - Setting the container width to occupy the full viewport width */

  .panel:nth-of-type(4),
  .panel:nth-of-type(5) {
    display: none;
  }
  /* Hiding the 4th and 5th panels when the viewport width is 480 pixels or less */
}