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CSS

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HTML 

              
                mixin layerR(n, c)
  if --n
    .layer(id=`layer${n}`)
      each type, position in {"zero":"number","one":"number","two":"number","three":"number","four":"number","five":"number","six":"number","seven":"number","eight":"number","nine":"number","plus":"operator","minus":"operator","multiply":"operator","equals":"operator"}
        if (position == "equals")
          input(type="radio" name=position class=position+' '+type).button
        else
          input(type="radio" name=`buttons${n}` class=position+' '+type).button
      .blank.button
      input(type="reset" class="reset" value="C" form="container").button
      if n == 1
        .layer.second-last
          input(type="radio" name="equals" class="equals operator").button
          input(type="reset" class="reset" value="C" form="container").button
          .layer.last
            input(type="reset" class="reset" value="C" form="container").button
            .answer
              - for (index=0;index<c;index++)
                .digit 
      +layerR(n, c)

mixin layer(n)
  +layerR(n + 1, n)
    
form.container(id="container")
  +layer(7)
!

CSS 

              
                @import url('https://fonts.googleapis.com/css2?family=Open+Sans&family=VT323&display=swap')
$buttons: ("zero":"0","one":"1","two":"2","three":"3","four":"4","five":"5","six":"6","seven":"7","eight":"8","nine":"9","plus":"+","minus":"-","multiply":"*","equals":"=")
@mixin layerR($n, $i, $c)
  @if $n != 0
    #layer#{$n}
      --checked#{$i}: initial
      --not-checked#{$i}: #{" "}
      --is-operator#{$i}: initial
      --is-not-operator#{$i}: #{" "}
      --is-number#{$i}: initial
      --expression#{$i}: + 0
      
      &>.number:checked ~ .layer
        --is-number#{$i}: #{" "}
      &>.operator
        &:not(.equals):checked ~ .layer
          --is-operator#{$i}: #{" "}
          --is-not-operator#{$i}: initial
      @each $name,$symbol in $buttons
        @if $name != "equals"
          &>.#{$name}
            &:checked ~ .layer
              --symbol#{$i}: #{$symbol} 
              --char#{$i}: "#{$symbol}"
              --checked#{$i}: #{" "}
              --not-checked#{$i}: initial
              
    @if $n == 1
      .answer
        $valid: ()
        $equation: ()
        @for $j from 1 to $c + 1
        
          --expression#{$j}: var(--is-operator#{$j - 1}, var(--is-operator#{$j}, var(--plus))) var(--symbol#{$j}) var(--is-operator#{$j}, var(--multiply) var(--power#{$j}))
        
          $power: ""
          $term: "var(--is-operator#{$c + 1})"
          @for $k from $j to $c + 1   
            $power: "var(--is-operator#{$c - ($k - $j) + 1}, var(--not-checked#{$c - ($k - $j) + 1}, var(--times-ten) #{$power}))"
            $term: "var(--is-operator#{$c - ($k - $j)}, var(--not-checked#{$c - ($k - $j)}, var(--expression#{$c - ($k - $j)}) #{$term}))"
          --power#{$j}: calc(#{"1 " + $power})
          --term#{$j}: calc(#{$term})

          --T2#{$j}: var(--is-operator#{$j - 1}) var(--term#{$j})
          --F2#{$j}: var(--is-operator#{$j - 1}, var(--blank))
          --term-or-blank#{$j}: var(--T2#{$j}, var(--F2#{$j}))

          --T3#{$j}: var(--is-operator#{$j}) var(--expression#{$j})
          --F3#{$j}: var(--is-operator#{$j}, var(--term-or-blank#{$j}))
          --final#{$j}: var(--T3#{$j}, var(--F3#{$j}))
          
          $equation: append($equation, var(--final#{$j}))
          $valid: append($valid, var(--is-not-operator#{$j}, var(--is-number#{$j - 1}) var(--is-number#{$j + 1})))

        --equation: #{$equation}
        --valid: #{$valid}

        &>.digit
          @for $i from 1 to $c + 1
            &:nth-child(#{$i})::before
              content: var(--submit, var(--char#{$i}))

    @include layerR($n - 1, $i + 1, $c)
    
@mixin layer($n)
  --blank: #{" "}
  --plus: +
  --multiply: *
  --times-ten: * 10
  --submit: initial
  --is-operator0: #{" "}
  --is-operator#{$n + 1}: #{" "}
  --is-number0: initial
  --is-number#{$n + 1}: initial
  @include layerR($n, 1, $n)
    
html
  width: 100vw
  height: 100vh
  body
    width: 100%
    height: 100%
    display: flex
    box-sizing: border-box
    *,*::before,*::after
      box-sizing: inherit

.container
  --width: 100vmin
  --break-point: clamp(0px, var(--width) - 500px, 50vmin)
  width: calc(var(--width) - var(--break-point) - 2%)
  height: calc((var(--width) - var(--break-point)) * (60/50))
  border: 1px solid black
  margin: auto
  position: relative
  overflow: hidden
  border-radius: 5%
  font-size: var(--width)
  @include layer(7)
            
.layer
  width: 100%
  height: 100%
  position: absolute
  display: grid
  grid-template-columns: repeat(4, 24%)
  grid-template-rows: repeat(5, 19.25%)
  grid-template-areas: "answer answer answer answer" "seven eight nine plus" "four five six minus" "one two three multiply" "blank zero reset equals"
  grid-gap: 1% 1.5%
  padding: 6% 3% 4%
  &:not(.container > .layer)
    pointer-events: none
    &::before
      opacity: 0
    & > .button
      opacity: 0
  &::before
    content: ''
    position: absolute
    width: 100%
    height: 100%
    background-color: white

.answer
  display: flex
  justify-content: flex-end
  align-items: center
  grid-area: answer
  border: 0.025em solid black
  padding: 0 1.5%
  max-width: 100%
  transform: translatey(-5%)
  font: calc((100% - var(--break-point)) * (28/100)) "VT323", monospace
  letter-spacing: min(0.04em, 4px)
  line-height: 0.5
  &::before
    counter-reset: convert calc(var(--equation))
    --placeholder: var(--submit, var(--checked1, "0"))
    --answer: var(--valid) var(--submit) counter(convert)
    --error: var(--valid, var(--submit) "ERROR")
    content: var(--placeholder) var(--answer, var(--error))
    margin-right: var(--valid, var(--submit) auto)
    transform: inherit
    
.digit
  flex: 0 1
  transform: inherit
    
.button
  width: 100% 
  height: 100%
  margin: 0
  border: 0.04em solid rgba(0,0,0,0.5)
  appearance: none
  font: calc((100% - var(--break-point)) * (11.5/100)) "Open Sans"
  display: flex
  justify-content: center
  align-items: center
  border-radius: 15%
  color: rgba(0,0,0,0.75)
  position: relative
  -webkit-tap-highlight-color: transparent
  user-select: none
  &.blank
    grid-area: blank
    background-color: rgba(0,0,0,0.25)
  &.reset
    grid-area: reset
    background-color: white
    padding: 0
    &::before
      content: 'foo'
      width: 100px
      height: 100px
      background: blue
      display: block
      position: absolute
      
  @each $name, $symbol in $buttons
    &.#{$name}
      grid-area: #{$name}
      &::before
        content: $symbol
  &.equals:checked 
    & ~ .reset
      box-shadow: inset 0 0 0 0.05em rgba(0,0,255,0.6)
    & ~ .layer
      & .last
        pointer-events: auto
      & .answer
        --submit: #{" "}
  &:not(.equals):checked
    opacity: 0
    & ~ .layer
      pointer-events: auto
      &:not(.second-last)::before
        opacity: 1
      & > .button
        opacity: 1
  &:not(.blank):not(.equals:checked):hover
    background-color: rgba(0,0,255,0.25)
    cursor: pointer
  &:focus
    outline: none
!

JS
!
999px

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