Make empty div of one line height

Some possibilities:

• Set height (or min-height) to the line-height‘s used value.

  The initial value of line-height is normal, whose used value is implementation-dependent, but the spec suggests a number between 1.0 and 1.2

  In my case (FF27 for winXP with font-family: "times new roman") that value is 1.25, so you could use height: 1.25em. However, that value might be different with other fonts or implementations.

  Then, it’s better to manually set line-height to some value, like line-height: 1.25em.

  div {
    border: 1px solid red;
    min-height: 1.25em;
    line-height: 1.25;
  }

  <div></div>

  Note that if you want to set those styles to the elements only when it has no content, you can use the :empty pseudo-class:

  div:empty {
    border: 1px solid red;
    height: 1.25em;
    line-height: 1.25;
  }

  <div></div>

• Inserting some content to the element.

  For example, you can add a normal space and set white-space to pre-wrap (or pre) to prevent the space from collapsing:

  div {
    border: 1px solid red;
    white-space: pre-wrap;
  }

  <div> </div>

  Alternatively, you can use a zero-width space (​)

  div { border: 1px solid red; }

  <div>​</div><!-- There is a zero-width space inside -->

  As you say,   would also work. However, it is a non-breaking space, so its purpose is preventing automatic line breaks. Then, using it would be semantically incorrect IMO.

  And as @BoltClock says, those whitespaces could be inserted with CSS pseudo-elements, but note that might not work on very old browsers.