wagtail/client/scss/components/forms/_title.scss

// Styles for title fields throughout the page.
// The `title` class is meant to be applied by end users throughout different forms APIs.
:where(.w-panel, .w-field).title {
  :where(input, textarea, .Draftail-Editor .public-DraftEditor-content) {
    @apply w-h2;
    color: theme('colors.text-context');
  }
}

// Styles for the title field when at the top of the page.
// It’s the first panel when comments are disabled, second if enabled.
.w-panel.title:nth-child(-n + 2) {
  .w-panel__header {
    @apply w-sr-only;

    .w-panel__anchor,
    .w-panel__toggle {
      @apply w-hidden;
    }
  }

  .w-field {
    display: grid;
  }

  // Force the help text after the field as the field also visually acts as page heading.
  .w-field__help {
    order: 2;
  }

  :is(input, textarea, .Draftail-Editor .public-DraftEditor-content) {
    @apply w-h1;
    color: theme('colors.text-context');
    // Slightly out-dented so the field’s is horizontally aligned.
    padding-inline-start: theme('spacing[1.5]');
    margin-inline-start: calc(-1 * theme('spacing[1.5]'));

    .w-density-snug & {
      font-size: theme('fontSize.26');
    }

    // Avoid calling attention to the field _unless_ it’s in one of those states.
    &:not(:hover, :focus, :placeholder-shown, [aria-invalid='true']) {
      // Hide w/ transparency to preserve border size and show it in forced-colors mode.
      border-color: transparent;
    }
  }
}