ash/webui/shimless_rma/resources/base_page.html

<style include="cr-shared-style shimless-rma-shared">
  #pageWrapper {
    align-items: stretch;
    display: flex;
    flex-direction: row;
    height: var(--content-container-height);
    width: 100%;
  }

  :host([equal-panes]) #leftPane {
    width: 50%;
  }

  :host([equal-panes]) #rightPane {
    width: 50%;
  }

  :host([left-pane-only]) #leftPane {
    margin-inline-end: 0;
    width: 100%;
  }

  :host([left-pane-only]) #rightPane {
    width: 0;
  }

  #leftPane {
    box-sizing: border-box;
    margin-inline-end: 80px;
    margin-top: 112px;
    width: 40%;
  }

  #rightPane {
    box-sizing: border-box;
    width: 60%;
  }
</style>
<div id="pageWrapper">
  <div id="leftPane">
    <slot name="left-pane"></slot>
  </div>
  <div id="rightPane">
    <slot name="right-pane"></slot>
  </div>
</div>