Tag HTML elements with their class names and IDs to visualize page structure

element-classes-and-ids-vanilla.css

*[class],
*[id] {
  position: relative;
  outline: 2px dashed red;
}

*[class]::before, *[class]::after,
*[id]::before,
*[id]::after {
  position: absolute;
  top: 0;
  left: 0;
  z-index: 1;
  padding: 0.25em;
  background-color: red;
  color: #fff;
  font-weight: normal;
  font-family: sans-serif;
  font-size: 0.9rem;
  border: 1px solid #fff;
}

*[class][class]::before,
*[id][class]::before {
  content: "." attr(class);
}

*[class][id]::after,
*[id][id]::after {
  content: "#" attr(id);
  right: 0;
  left: auto;
  background-color: blue;
}

*[class][id],
*[id][id] {
  outline-color: blue;
}

element-classes-and-ids.scss

*[class],
*[id] {
  position: relative;
  outline: 2px dashed red;

  &::before,
  &::after {
    position: absolute;
    top: 0;
    left: 0;
    z-index: 1;
    padding: 0.25em;
    background-color: red;
    color: #fff;
    font-weight: normal;
    font-family: sans-serif;
    font-size: .9rem;
    border: 1px solid #fff;
  }

  &[class]::before {
    content: "." attr(class);
  }

  &[id]::after {
    content: "#" attr(id);
    right: 0;
    left: auto;
    background-color: blue;
  }
  
  &[id] {
    outline-color: blue;
  }
}