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CSS

dont understand

Use the @else directive to add a condition that will print an error message if we pass an invalid argument when including this mixin.

idk what to do

2 Answers

Or this:

@mixin child($pos) {
  @if $pos == "first" {
    &:#{$pos}-child {
      float: left;
    }
  } 
  @else if $pos == "last" {
    &:#{$pos}-child {
      float: right;
    }
  }
  @else {
   @error 'That argument is invalid.';
 }
}

Try something like:

@else {
   @error 'That argument is invalid.';
}