Using border以及:after to achieve, in order to facilitate the viewing effect, the following demo line width is set to 2px.
Online preview

html >>>>>>

<p class='treeline'></p> 

css >>>>>>

.treeline {
  display: inline-block;
  width: 100px;
  height: 26px;
  padding: 12px 0;
  box-sizing: border-box;
  border-left: 2px solid #888;
}
.treeline:after {
  content: '';
  display: block;
  width: 100%;
  height: 100%;
  background: #888;
}

Use border for vertical lines and before for horizontal lines

    .list-ui{
      position: relative;
      margin-left: 5px;
      padding-left: 45px;
      border-left: 1px solid #ccc;
    }
    .list-ui:before{
      position: absolute;
      top: 50%;
      left: 0;
      margin-top: -0.5px;
      content: '';
      display: block;
      width: 40px;
      height: 1px;
      background: #ccc;
    }

I have a bold idea~, use ::before and ::after to combine them

Declare a class border and write a horizontal line. Use a pseudo class to write a horizontal line and then rotate it 90 degrees and move the position. Finished
Then this class can be reused~~