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HTML 

              
                <input class="c-checkbox" type="checkbox" id="start">
<input class="c-checkbox" type="checkbox" id="progress2">
<input class="c-checkbox" type="checkbox" id="progress3">
<input class="c-checkbox" type="checkbox" id="finish">
<div class="c-form__progress"></div>

<div class="c-formContainer">
  <div class="c-welcome">Welcome aboard!</div>
  <form class="c-form" action="">
    <div class="c-form__group">
      <label class="c-form__label" for="username">
                    <input
                        type="text"
                        id="username"
                        class="c-form__input"
                        placeholder=" "
                        pattern="[^\s]+"
                        required>

                    <label class="c-form__next" for="progress2" role="button">
                        <span class="c-form__nextIcon"></span>
                    </label>

      <span class="c-form__groupLabel">Create your username.</span>
      <b class="c-form__border"></b>
      </label>
    </div>

    <div class="c-form__group">
      <label class="c-form__label" for="femail">
                    <input
                        type="email"
                        id="femail"
                        class="c-form__input"
                        placeholder=" "
                        pattern="[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{1,63}$"
                        required>

                    <label class="c-form__next" for="progress3" role="button">
                        <span class="c-form__nextIcon"></span>
                    </label>

      <span class="c-form__groupLabel">What's your email?</span>
      <b class="c-form__border"></b>
      </label>
    </div>

    <div class="c-form__group">
      <label class="c-form__label" for="fpass">
                    <input
                        type="password"
                        id="fpass"
                        class="c-form__input"
                        placeholder=" "
                        required>

                    <label class="c-form__next" for="finish" role="button">
                        <span class="c-form__nextIcon"></span>
                    </label>

      <span class="c-form__groupLabel">Create your password.</span>
      <b class="c-form__border"></b>
      </label>
    </div>

    <label class="c-form__toggle" for="start">Register<span class="c-form__toggleIcon"></span></label>
  </form>
</div>
!

CSS 

              
                $black: #000000;
$blue-ribbon: #0069ec;
$dodger-blue: #25a3ff;
$mine-shaft: #333333;
$scorpion: #565656;
$torch-red: #ff0033;
$white: #ffffff;

@mixin form-group-visible {
  visibility: visible;
  opacity: 1;

  .c-form__border {
    width: 100%;
  }
}

@mixin form-group-hidden {
  visibility: hidden;
  opacity: 0;

  .c-form__border {
    width: 0;
  }
}

@mixin input-listener {
  // hints user for invalid input
  .c-form__input:not(:placeholder-shown):invalid ~  {
    .c-form__border,
    .c-form__next {
      color: $torch-red;
    }
  }

  // allows submit
  .c-form__input:required:valid ~  {
    .c-form__next {
      pointer-events: initial;
      color: $scorpion;
    }
  }
}

body {
  font-family: "Open Sans", sans-serif;
  margin: 0;
  height: 100vh;
  display: grid;
  place-items: center;
  background-color: $dodger-blue;
  position: relative;
  overflow: hidden;
}

.c-checkbox {
  display: none;
}

// opens first form
.c-checkbox:nth-of-type(1):checked ~ .c-formContainer {
  .c-form__toggle {
    visibility: hidden;
    opacity: 0;
    transform: scale(0.7);
  }

  .c-form {
    width: 382px;

    .c-form__group:nth-child(1) {
      @include form-group-visible;
      @include input-listener;
    }
  }
}

// opens second form
.c-checkbox:nth-of-type(2):checked ~  {
  .c-form__progress {
    width: calc(100vw / 3);
  }

  .c-formContainer .c-form {
    .c-form__group:nth-child(1) {
      @include form-group-hidden;
    }

    .c-form__group:nth-child(2) {
      @include form-group-visible;
      @include input-listener;
    }
  }
}

// opens third form
.c-checkbox:nth-of-type(3):checked ~  {
  .c-form__progress {
    width: calc((100vw / 3) + 100vw / 3);
  }

  .c-formContainer .c-form {
    .c-form__group:nth-child(2) {
      @include form-group-hidden;
    }

    .c-form__group:nth-child(3) {
      @include form-group-visible;
      @include input-listener;
    }
  }
}

#finish:checked ~  {
  .c-form__progress {
    width: 100vw;
  }

  .c-formContainer {
    .c-form {
      transition: opacity 0.2s 0.1s, transform 0.3s;
      opacity: 0;
      transform: translateX(50%) scaleX(0);

      .c-form__group:nth-child(3) {
        transition: 0s;
        @include form-group-hidden;
      }
    }

    .c-welcome {
      opacity: 1;
    }
  }
}

.c-formContainer {
  width: 180px;
  height: 65px;
  z-index: 1;

  // sets transition-origin center for .c-form width
  display: flex;
  justify-content: center;
}

.c-welcome {
  position: absolute;
  width: max-content;
  height: inherit;

  font-size: 40px;
  color: $white;
  opacity: 0;
  transition: 0.3s;
}

.c-form {
  position: relative;
  width: inherit;
  height: inherit;
  background-color: $white;
  box-shadow: 0 5px 10px -2px rgba($black, 0.2),
    0 2px 4px -1px rgba($black, 0.3);
}

.c-form__toggle {
  width: inherit;
  height: inherit;

  font-size: 18px;
  color: $mine-shaft;
  cursor: pointer;
  transition: 0.2s;

  // centers text
  display: flex;
  align-items: center;
  justify-content: center;
}

.c-form__toggleIcon {
  display: inline-block;
  margin-left: 10px;
  position: relative;
  width: 15px;
  height: 17px;

  &::before,
  &::after {
    content: "";
    position: absolute;
    background-color: $scorpion;
  }

  // head
  &::before {
    left: 50%;
    transform: translateX(-50%);
    width: 9px;
    height: 9px;
    border-radius: 50%;
  }

  // body
  &::after {
    bottom: 0;
    width: 100%;
    height: 7px;
    border-radius: 7px 7px 0 0;
  }
}

.c-form__group {
  width: 100%;
  height: 100%;

  visibility: hidden;
  opacity: 0;
  padding: 12px 15px;
  box-sizing: border-box;
  transition: 0.2s 0.2s;

  // allows to overlap forms
  position: absolute;
}

.c-form__label {
  position: relative;
  cursor: pointer;

  // 40px = button size
  width: calc(100% - 40px);
  height: 100%;

  // positions .c-form__input and button
  display: flex;
  align-items: flex-end;
}

.c-form__input {
  font-size: 20px;
  font-family: inherit;

  width: 100%;
  height: 90%;
  border: 0;
  outline: 0;
  color: $mine-shaft;
  box-sizing: border-box;
  cursor: pointer;

  &:focus,
  &:not(:placeholder-shown) {
    ~ .c-form__groupLabel {
      font-size: 10px;
      top: -4px;
      transform: translateY(0);
      color: $mine-shaft;
    }

    ~ .c-form__border,
    ~ .c-form__next {
      transition: 0.3s;
    }
  }

  &:focus {
    cursor: initial;
  }
}

.c-form__groupLabel {
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
  transition: 0.2s;
}

.c-form__border {
  position: absolute;
  width: 0;
  height: 3px;
  color: $dodger-blue;
  background-color: currentColor;
  transition: 1s 0.2s ease-out;
}

.c-form__next {
  color: $dodger-blue;
  position: absolute;
  right: -40px;
  height: 100%;
  width: 40px;
  cursor: pointer;
  pointer-events: none;

  display: flex;
  align-items: center;
  justify-content: flex-end;
}

.c-form__nextIcon {
  position: relative;
  right: 1.5px;
  width: 20px;
  height: 3px;
  background-color: currentColor;

  &::before,
  &::after {
    content: "";
    position: absolute;
    right: -1.5px;
    width: 15px;
    height: 3px;
    background-color: inherit;
  }

  &::before {
    bottom: -1.5px;
    transform-origin: top right;
    transform: rotate(45deg);
  }

  &::after {
    top: -1.5px;
    transform-origin: bottom right;
    transform: rotate(-45deg);
  }
}

.c-form__progress {
  position: fixed;
  left: 0;
  height: 100%;
  width: 0;
  background-color: $blue-ribbon;
  transition: 0.3s;
}
!

JS 

              
                // Yea yea yea, I know semantic markups and I know click events are should be done on JavaScript. I'm just trying to have fun here. :)

// P.S. No space for username pleassse...
!
999px

Console