just-the-docs/node_modules/primer-support/lib/mixins/typography.scss

// Text hiding for image based text replacement.
// Higher performance than -9999px because it only renders
// the size of the actual text, not a full 9999px box.
@mixin hide-text() {
  overflow: hidden;
  text-indent: 100%;
  white-space: nowrap;
}

// Heading mixins for use within components
// These match heading utilities in utilities/typography
@mixin h1 {
  font-size: $h1-size;
  font-weight: $font-weight-bold;
}

@mixin h2 {
  font-size: $h2-size;
  font-weight: $font-weight-bold;
}

@mixin h3 {
  font-size: $h3-size;
  font-weight: $font-weight-bold;
}

@mixin h4 {
  font-size: $h4-size;
  font-weight: $font-weight-bold;
}

@mixin h5 {
  font-size: $h5-size;
  font-weight: $font-weight-bold;
}

@mixin h6 {
  font-size: $h6-size;
  font-weight: $font-weight-bold;
}

// Responsive heading mixins
// There are no responsive mixins for h4—h6 because they are small
// and don't need to be smaller on mobile.
@mixin f1-responsive {
  font-size: $h1-size-mobile;

  // 32px on desktop
  @include breakpoint(md) { font-size: $h1-size; }

}

@mixin f2-responsive {
  font-size: $h2-size-mobile;

  // 24px on desktop
  @include breakpoint(md) { font-size: $h2-size; }
}

@mixin f3-responsive {
  font-size: $h3-size-mobile;

  // 20px on desktop
  @include breakpoint(md) { font-size: $h3-size; }

}

// These use the mixins from above for responsive heading sizes.
// The following mixins can be used where it's convenient or necessary to
// couple the responsive font-size with the font-weight.
@mixin h1-responsive {
  @include f1-responsive;
  font-weight: $font-weight-bold;
}

@mixin h2-responsive {
  @include f2-responsive;
  font-weight: $font-weight-bold;
}

@mixin h3-responsive {
  @include f3-responsive;
  font-weight: $font-weight-bold;
}