<div class="main">
  <div class="block1">Невидимый</div>
</div>
<div class="main toggle">
  <div class="block1">Видимый</div>
</div>

.block1
  padding: 1rem
  border: 2px solid red

.main
  .block1
    display: none

  &.toggle
    .block1
      display: flex
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