chrome/browser/resources/compose/textarea.html

<style>
  :host {
    --compose-textarea-input-height: calc(
        4lh + (2 * var(--compose-textarea-input-padding)));
    --compose-textarea-input-padding: 10px;
    --compose-textarea-readonly-height: 48px;
    --compose-textarea-readonly-padding: 16px;
    --compose-textarea-readonly-background:
        var(--color-compose-dialog-textarea-readonly-background);

    display: grid;
    grid-template-areas: "content";
    align-items: start;
    grid-auto-columns: 100%;
  }

  #inputContainer,
  #readonlyContainer {
    grid-area: content;
    width: 100%;
    box-sizing: border-box;
  }

  #inputContainer {
    display: flex;
    flex-direction: column;
    height: 100%;
  }

  textarea {
    appearance: none;
    background: transparent;
    border: none;
    outline: solid 1px var(--color-compose-dialog-textarea-outline);
    outline-offset: -1px;
    border-radius: 8px;
    color: var(--color-compose-dialog-textarea);
    font: inherit;
    font-size: 12px;
    line-height: 16px;
    resize: none;
    width: 100%;
    height: var(--compose-textarea-input-height);
    box-sizing: border-box;
    padding: 10px;
    scroll-padding-block: 10px;
  }

  :host([readonly]) textarea {
    overflow: hidden;
  }

  textarea::placeholder {
    color: var(--color-compose-dialog-textarea-placeholder);
  }

  #input:focus::placeholder  {
    color: transparent;
  }

  :host([invalid-input_]) textarea {
    outline: solid 2px var(--color-compose-dialog-error);
    outline-offset: -2px;
  }

  .error {
    margin: 4px 10px 0;
    color: var(--color-compose-dialog-error);
    font-size: 11px;
    line-height: 16px;
    user-select: text;
  }

  textarea:focus {
    outline: solid 2px var(--cr-focus-outline-color);
    outline-offset: -2px;
  }

  #readonlyContainer {
    border-radius: 8px;
    display: flex;
    padding: 16px;
    background: var(--color-compose-dialog-textarea-readonly-background);
    color: var(--color-compose-dialog-textarea-readonly-foreground);
  }

  #readonlyText {
    font-size: 12px;
    font-weight: 400;
    line-height: 16px;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
    width: 100%;
    user-select: text;
  }

  #editButtonContainer {
    --cr-icon-button-icon-size: var(--cr-icon-size);
    width: var(--cr-icon-size);
    height: 16px;
    display: flex;
    align-items: center;
    justify-content: center;
    margin-inline-start: 16px;
  }

  cr-icon-button {
    --cr-icon-button-fill-color: var(--color-compose-dialog-foreground-subtle);
    margin: 0;
  }

  /**
   * cr-hidden-style stamps an !important on [hidden] elements, which prevents
   * any animations/transitions from applying. Instead of using cr-hidden-style,
   * list all the [hidden] elements below. Not all elements need this, just
   * the ones that override the default 'display' property.
   */
  #inputContainer[hidden],
  #readonlyContainer[hidden],
  #editButtonContainer[hidden] {
    display: none;
  }
</style>

<div id="inputContainer" hidden$="[[readonly]]">
  <textarea id="input"
      placeholder="$i18n{inputPlaceholderTitle}
 • $i18n{inputPlaceholderLine1}
 • $i18n{inputPlaceholderLine2}
 • $i18n{inputPlaceholderLine3}"
      value="{{value::input}}"
      required
      autofocus
      aria-invalid="[[invalidInput_]]"
      aria-errormessage="error"
      on-change="onChangeTextArea_">
  </textarea>
  <div id="error" class="error" role="region" aria-live="assertive"
      hidden$="[[!invalidInput_]]">
    <div id="tooShortError" hidden$="[[!tooShort_]]">$i18n{errorTooShort}</div>
    <div id="tooLongError" hidden$="[[!tooLong_]]">$i18n{errorTooLong}</div>
  </div>
</div>

<div id="readonlyContainer" hidden$="[[!readonly]]">
  <div id="readonlyText">[[value]]</div>
  <div id="editButtonContainer"
      hidden$="[[!shouldShowEditIcon_(allowExitingReadonlyMode, readonly)]]">
    <cr-icon-button id="editButton" iron-icon="compose:edit"
        title="$i18n{editButton}" on-click="onEditClick_">
    </cr-icon-button>
  </div>
</div>