why does font-family: monospace changes the layout of my

Question

I am using some code from this answer on a question of how to align dd and dt elements on the same line and the results are good.

The following code:

 dl {
   width: 100%;
   overflow: hidden;
   background:#ff0;
   padding: 0;
   margin: 0;
 }
 dt {
   float: left;
   text-align: right;
   width: 20%;
   background: #cc0;
   padding: 0;
   margin: 0;

   font-weight: bold;
   /* Question 2 */
   /* font-family: monospace;  */
   margin-right: 10%;
 }
 dd {
   float: left;
   text-align: left;
   width: 70%;
   background: #dd0;
   padding: 0;
   margin: 0;
   /* Question 1 */
   /* font-family: monospace; */
 }

<dl>
  <dt>cat</dt><dd>small, domesticated  feline</dd>
  <dt>bear</dt><dd>large omnivore (top land predator)</dd>
  <dt>giraffe</dt><dd>the most useless quadruped</dd>
</dl>

... produces this image:

However, when I try to apply font-family:monospace to either the dd or the dt elements the layout breaks.

E.g. if I remove the comment below the Question 1 line this is what I see:

Whereas if I remove the comment below the Question 2 line I get a less dramatic failure but still not pleasant to look at:

I would like to understand:

1. why font-family: monospace can upset the layout so much (particularly for the dd element)
2. how to fix that

Answer 1

The reason is the different line-height of the fonts used. Your system's monospace font is slightly smaller than the other font used. This leads to different heights of the dd and dt elements. Now, in the first case (dd is monospace), the second dt element is below the first dd because there is like literally one pixel left of the first dt's height, so it gets placed there due to the floating.

If both are set to monospace, it looks good again because all elements have the same line-height again.

 dl {
   width: 100%;
   overflow: hidden;
   background:#ff0;
   padding: 0;
   margin: 0;
 }
 dt {
   float: left;
   text-align: right;
   width: 20%;
   background: #cc0;
   padding: 0;
   margin: 0;

   font-weight: bold;
   /* Question 2 */
   font-family: monospace;
   margin-right: 10%;
 }
 dd {
   float: left;
   text-align: left;
   width: 70%;
   background: #dd0;
   padding: 0;
   margin: 0;
   /* Question 1 */
   font-family: monospace;
 }

<dl>
  <dt>cat</dt><dd>small, domesticated  feline</dd>
  <dt>bear</dt><dd>large omnivore (top land predator)</dd>
  <dt>giraffe</dt><dd>the most useless quadruped</dd>
</dl>

Alternatively, you can also explicitly set a line-height if you only want to have one of the two elements to use monospace font:

 dl {
   width: 100%;
   overflow: hidden;
   background:#ff0;
   padding: 0;
   margin: 0;
 }
 dt {
   float: left;
   text-align: right;
   width: 20%;
   background: #cc0;
   padding: 0;
   margin: 0;

   font-weight: bold;
   /* Question 2 */
   /* font-family: monospace; */
   margin-right: 10%;
 }
 dd {
   float: left;
   text-align: left;
   width: 70%;
   background: #dd0;
   padding: 0;
   margin: 0;
   /* Question 1 */
   font-family: monospace;
 }
 dd, dt {
   line-height: 14px;
 }

<dl>
  <dt>cat</dt><dd>small, domesticated  feline</dd>
  <dt>bear</dt><dd>large omnivore (top land predator)</dd>
  <dt>giraffe</dt><dd>the most useless quadruped</dd>
</dl>

Answer 2

Try giving font family to dl tag like this, if you want monospace for the entire content.

 dl {
   width: 100%;
   overflow: hidden;
   background:#ff0;
   padding: 0;
   margin: 0;
   font-family: monospace;
 }
 dt {
   float: left;
   text-align: right;
   width: 20%;
   background: #cc0;
   padding: 0;
   margin: 0;
   font-weight: bold;
   /* Question 2 */
   /* font-family: monospace; */
   margin-right: 10%;
 }
 dd {
   float: left;
   text-align: left;
   width: 70%;
   background: #dd0;
   padding: 0;
   margin: 0;
   /* Question 1 */
   /* font-family: monospace; */
 }

<dl>
  <dt>cat</dt><dd>small, domesticated  feline</dd>
  <dt>bear</dt><dd>large omnivore (top land predator)</dd>
  <dt>giraffe</dt><dd>the most useless quadruped</dd>
</dl>