How to draw the reference lines(blue Auxiliary lin…

Auxiliary line

How to draw the blue dashed reference lines with css like the one in firefox development?

Answer

I finally build references line like this : to make the parent div’s top,left, width,height variables, and calc them to lines.

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      body {
        margin: 0;
      }
      :root {
        --left: 30px;
        --top: 20px;
        --width: 100px;
        --height: 60px;
      }
      .mydiv {
      
        width: var(--width);
        height: var(--height);
        left: var(--left);
        top: var(--top);
        position: relative;
        background-color: black;
      }
      .up {
        border-top: 1px dashed aqua;
        height: 1px;
        left: calc(0px - var(--left));
        line-height: 1px;
        width: 100vw;
      }
      .down {
        border-top: 1px dashed aqua;
        height: 1px;
        left: calc(0px - var(--left));
        top: var(--height);
        width: 100vw;
      }
      .left {
        border-left: 1px dashed aqua;
        width: 1px;
        top: calc(0px - var(--top));
        height: 100vh;
      }
      .right {
        border-left: 1px dashed aqua;
        width: 1px;
        top: calc(0px - var(--top));
        left: var(--width);
        height: 100vh;
      }
      .v-line {
        position: absolute;
      }
    </style>
  </head>
  <body>
    <div class="mydiv">
      <span class="v-line up"></span>
      <span class="v-line down"></span>
      <span class="v-line left"></span>
      <span class="v-line right"></span>
    </div>
  </body>
</html>

If you have more good idea pleace leave your answer.

How to draw the reference lines(blue Auxiliary lines) with css like the one in firefox devtools?

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