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HTML

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CSS

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CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

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About Vendor Prefixing

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                <div id="parasite"></div>
!

CSS 

              
                // Blinking light warning

// #divtober 17: Parasite
// a.singlediv.com/divtober2020

black(alpha)
  rgba(0,0,0,alpha)

white(alpha)
  rgba(255,255,255,alpha)

body
  min-height: 400px
  height: 100vh
  display: grid
  place-content: center
  background-color: #111
  z-index: 1
  overflow: hidden

div
  position: relative

  &:before,
  &:after
    display: block
    content: ''
    position: absolute
    
  @media (max-width: 400px)
    &:not(.no-scale)
      transform: scale(.8)

#parasite
  width: 26em
  height: 400px
  background-repeat: no-repeat
  background-image: linear-gradient(to top, darken(sienna,10) .5em, alpha(sienna,0) 2.5em),
                    linear-gradient(to left, alpha(palegoldenrod,.05) .2em, transparent .2em),
                    radial-gradient(ellipse at 100% 100%, alpha(palegoldenrod,.4) 20%, alpha(palegoldenrod,0) 60%),
                    radial-gradient(ellipse at 60% 0, alpha(palegoldenrod,.3) 20%, alpha(palegoldenrod,0) 60%)
  background-size: .2em 54%,
                   100% 100%,
                   28em 50%,
                   23em 70%
  background-position: 10.3em 0,
                       100% 0,
                       6em 80%,
                       -6em 0
  animation: parasite 11000ms infinite ease-in
  animation-delay: 3000ms
  @media (prefers-reduced-motion)
    animation: none
  @media screen and (max-width: 400px)
    margin-left: -5em

  &:before
    width: 4.8em
    height: @width
    left: 8em
    top: 54%
    background-repeat: no-repeat
    background-color: alpha(palegoldenrod,.45)
    background-image: radial-gradient(ellipse, white 15%,
                                               alpha(palegoldenrod,.8) 23%,
                                               alpha(palegoldenrod, 0) 45%),
                      radial-gradient(circle, white 15%,
                                              alpha(palegoldenrod,.8) 25%,
                                              alpha(palegoldenrod,0) 40%),
                      radial-gradient(ellipse, alpha(palegoldenrod,.9) 25%,
                                               alpha(palegoldenrod, 0) 50%),
                      radial-gradient(ellipse, alpha(palegoldenrod,.5) 25%,
                                               alpha(palegoldenrod, 0) 50%),
                      radial-gradient(ellipse, alpha(palegoldenrod,.9) 10%,
                                               alpha(palegoldenrod,0) 30%)
    background-size: 40% 60%,
                     100% 70%,
                     80% 30%,
                     100% 80%,
                     60% 40%
    background-position: 50% 80%,
                         50% -15%,
                         50% 0%,
                         50% -20%,
                         50% 135%
    border-radius: 50%
    box-shadow: inset 0 4.2em 0 -4em darken(sienna,10),
                inset 0 .4em .3em khaki,
                inset 0 -.3em .3em alpha(sienna,.6),
                0 .2em 2em .3em alpha(palegoldenrod,.4)

  &:after
    width: 100%
    height: 100%
    background-image: linear-gradient(160deg, #111 14em,
                                              transparent 14em,
                                              transparent 23em,
                                              #111 23em)

@keyframes parasite
  0%, 8.4%
    opacity: 0
  9.6%, 16.8%
    opacity: 1
  18%, 20.4%
    opacity: 0
  21.6%, 23.8%
    opacity: 1
  25%, 27.4%
    opacity: 0
  29.6%, 32%
    opacity: 1
  33.2%, 40.4%
    opacity: 0
  41.6%, 44%
    opacity: 1
  45.2%, 47.6%
    opacity: 0
  48.8%, 51.2%
    opacity: 1
  52.4%, 59.6%
    opacity: 0
  60.8%, 63.2%
    opacity: 1
  64.4%, 66.8%
    opacity: 0
  68%, 70.4%
    opacity: 1
  71.6%, 74%
    opacity: 0
  76.2%, 78.6%
    opacity: 1
  79.8%, 82.2%
    opacity: 0
  83.4%, 90.6%
    opacity: 1
  91.8%, 100%
    opacity: 0
!

JS 

              
                // @lynnandtonic
// a.singlediv.com
!
999px

Console