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HTML

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Insecure Resource

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CSS

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CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                .envelope
  input.envelope__check(type='checkbox')
  .envelope__flap.envelope__flap--inside
  .envelope__flap
  .envelope__letter
    .letter
      .letter__content
        p Hey! 👋
        p.
          The whole purpose of places like Starbucks is for people with no decision-making ability whatsoever to make six decisions just to buy one cup of coffee. Short, tall, light, dark, caf, decaf, low-fat, non-fat, etc. 😕
      .letter__content
      .letter__content
        p.
          So people who don't know what the hell they're doing or who on earth they are can, for only $2.95, get not just a cup of coffee but an absolutely defining sense of self: Tall. Decaf. Cappuccino. ☕️
        span.sign-off Joe Fox
  .envelope__back
!

CSS 

              
                *
  box-sizing border-box

body
  background-color #111
  text-align       center
  height 100vh
  width 100vw
  display flex
  align-items center
  justify-content center
  overflow hidden
  margin 0
  padding 0
  font-family 'Arial', sans-serif
  font-size 12px


$flapTransition = 0.25s
$contentTransition = .25s
$foldTransition = .25s
$envelopeColor = #cebda6
$patternGap = 10px
$borderWhite = $envelopeColor
$borderRed = #e74c3c
$borderBlue = dodgerblue

.envelope
  width 300px
  height 150px
  background darken($envelopeColor, 20%)
  position relative

  &__check
    height 100%
    width 100%
    z-index 10
    opacity 0
    position absolute
    left 0
    top 0
    margin 0
    cursor pointer

    &:checked
      & ~ .envelope__flap
        transition transform $flapTransition 0s, z-index 0s $flapTransition
        transform rotateX(-180deg)
        z-index 0

      & ~ .envelope__letter
        transform translateY(-110%)
        transition transform $contentTransition $flapTransition, z-index 0s ($flapTransition +$contentTransition)
        z-index 5

        .letter__content:nth-child(2)
        .letter__content:nth-child(3)
          transition transform $foldTransition ($flapTransition + $contentTransition)
          transform rotateX(0deg)

  &__flap
    $borderWhite = darken($envelopeColor, 10%)
    position absolute
    top 0
    left 0
    right 0
    height 75%
    transition transform $flapTransition ($contentTransition + $foldTransition), z-index 0s ($contentTransition + $foldTransition)
    transform-origin top center
    backface-visibility hidden
    background repeating-linear-gradient(-45deg, $borderWhite, $borderWhite $patternGap, $borderRed $patternGap, $borderRed ($patternGap * 2px), $borderWhite ($patternGap * 2px), $borderWhite ($patternGap * 3px), $borderBlue ($patternGap * 3px), $borderBlue ($patternGap * 4px)) 0 0 / 100% 5px no-repeat,
      darken($envelopeColor, 10%)
    z-index 4
    // clip-path polygon(5% 40%, 30% 100%, 70% 100%, 95% 40%, 100% 0, 0 0)
    clip-path polygon(5% 40%, 50% 100%, 95% 40%, 100% 0, 0 0)

    &--inside
      background darken($envelopeColor, 10%)
      backface-visibility visible

  &__back
    height 100%
    width 100%
    height 100%
    width 100%
    position absolute
    top 0
    left 0
    z-index 3
    background repeating-linear-gradient(-45deg, $borderWhite, $borderWhite $patternGap, $borderRed $patternGap, $borderRed ($patternGap * 2px), $borderWhite ($patternGap * 2px), $borderWhite ($patternGap * 3px), $borderBlue ($patternGap * 3px), $borderBlue ($patternGap * 4px)) 0 100% / 100% 5px no-repeat,
      repeating-linear-gradient(-45deg, $borderWhite, $borderWhite $patternGap, $borderRed $patternGap, $borderRed ($patternGap * 2px), $borderWhite ($patternGap * 2px), $borderWhite ($patternGap * 3px), $borderBlue ($patternGap * 3px), $borderBlue ($patternGap * 4px)) 0 100% / 5px 100% no-repeat,
      repeating-linear-gradient(-45deg, $borderWhite, $borderWhite $patternGap, $borderRed $patternGap, $borderRed ($patternGap * 2px), $borderWhite ($patternGap * 2px), $borderWhite ($patternGap * 3px), $borderBlue ($patternGap * 3px), $borderBlue ($patternGap * 4px)) 100% 100% / 5px 100% no-repeat,
      $envelopeColor
    clip-path polygon(80% 40%, 100% 0, 100% 100%, 0 100%, 0 0, 20% 40%)

    &:after
      $borderWhite = darken($envelopeColor, 5%)
      height 100%
      width 100%
      background repeating-linear-gradient(-45deg, $borderWhite, $borderWhite $patternGap, $borderRed $patternGap, $borderRed ($patternGap * 2px), $borderWhite ($patternGap * 2px), $borderWhite ($patternGap * 3px), $borderBlue ($patternGap * 3px), $borderBlue ($patternGap * 4px)) 0 100% / 100% 5px no-repeat,
        darken($envelopeColor, 5%)
      content ''
      position absolute
      top 0
      left 0
      clip-path polygon(70% 10%, 100% 100%, 0 100%, 30% 10%)

  &__letter
    height 90%
    width 90%
    z-index 1
    position absolute
    left 5%
    top 5%
    transition transform $contentTransition $foldTransition, z-index 0s $foldTransition
    transform translateY(0)

.letter
  &__content
    height 100%
    width 100%
    background-color white
    top 0
    left 0
    position absolute
    padding 14px
    text-align justify

    &:nth-child(2)
    &:nth-child(3)
      transition transform $foldTransition
      top 100%
      transform-origin top center
      transform rotateX(180deg)
      backface-visibility visible
      z-index 2
      border-top 1px solid #eee
    
    &:nth-child(3)
      backface-visibility hidden

.sign-off
  position absolute
  bottom 12px
  right 12px
  font-size 20px
  font-family cursive
  transform skewX(-20deg)
!

JS
!
999px

Console