chrome/browser/resources/read_later/side_panel/bookmark_folder.html

<style>
  :host {
    display: block;
    user-select: none;
    white-space: nowrap;
  }

  .row {
    align-items: center;
    appearance: none;
    background: transparent;
    border: none;
    box-sizing: border-box;
    color: currentColor;
    cursor: pointer;
    display: grid;
    grid-template-areas: 'arrow icon title';
    grid-template-columns: 20px 20px auto;
    height: 40px;
    line-height: 40px;
    padding-inline-start: calc(var(--node-depth) * 17px);
    text-align: start;
    width: 100%;
  }

  .row:hover,
  .row:focus-visible:focus {
    background-color: var(--cr-hover-background-color);
    outline: none;
  }

  .row:active {
    background-color: var(--cr-active-background-color);
  }

  #arrow {
    grid-area: arrow;
    justify-self: center;
  }

  #arrowIcon {
    --cr-icon-button-hover-background-color: transparent;
    --cr-icon-button-active-background-color: transparent;
    margin: 0;
    transform: rotate(-90deg);
    transition: transform 100ms cubic-bezier(0.4, 0, 0.2, 1);
  }

  #arrowIcon[open] {
    transform: rotate(0);
  }

  :host-context([dir='rtl']) #arrowIcon:not([open]) {
    transform: rotate(90deg);
  }

  .icon {
    background-position: left center;
    background-repeat: no-repeat;
    grid-area: icon;
    height: 16px;
    justify-self: center;
    width: 16px;
  }

  #folderIcon {
    background-image: url(chrome://theme/IDR_FOLDER_CLOSED);
  }

  .title {
    grid-area: title;
    overflow: hidden;
    padding: 0 10px;
    text-overflow: ellipsis;
  }

  .bookmark {
    color: currentColor;
    text-decoration: none;
  }
</style>
<button class="row" on-click="onFolderClick_" tabindex="0">
  <div id="arrow" hidden$="[[!folder.children.length]]">
    <cr-icon-button
        id="arrowIcon"
        iron-icon="cr:arrow-drop-down"
        open$="[[open_]]"
        tabindex="-1">
    </cr-icon-button>
  </div>
  <div id="folderIcon" class="icon"></div>
  <div class="title">[[folder.title]]</div>
</button>

<div id="children" role="group">
  <template is="dom-if" if="[[open_]]">
    <template is="dom-repeat" items="[[folder.children]]">
      <template is="dom-if" if="[[!item.url]]">
        <bookmark-folder role="treeitem" folder="[[item]]"
            depth="[[childDepth_]]"
            open-folders="[[openFolders]]">
        </bookmark-folder>
      </template>

      <template is="dom-if" if="[[item.url]]">
        <a role="treeitem" href="[[item.url]]" class="bookmark row"
            on-click="onBookmarkClick_">
          <div
              class="icon"
              style="background-image: [[getBookmarkIcon_(item.url)]]">
          </div>
          <div class="title">[[item.title]]</div>
        </a>
      </template>
    </template>
  </template>
</div>