Smaller EM placed inside P generates bigger box


Trying to keep vertical rhythm but slowly giving up. Firefox 3.6.9, Linux.
Please take a look at

LI contains two P. Both have line-height set to 21px. Despite the fact that em.comment-data has font-size of 10px (line-height of 21px) it cause the whole paragraph is not 21px but 23px. I removed it temporarily and the problem vanished. It also turns out that setting its size to 14px helps.

Where’s the problem? Can’t figure it out myself. :frowning:

Yup, I know that some parents extend their box to accomodate their children.
Heck, but I don’t understand this:

there are two EMs in first P. First EM’s height evaluates to 18px. Second to 13px. But the latter makes P to extend its evaluated height to 22.5px (its size is set to 10px). What is this 1.5px from?

and you’re welcome :slight_smile: you should practice using developer tools browsers have to offer. firebug is really good.

Thanks. :slight_smile: I use Firebug in my all projects. I’m studying typography these days so I wanted to improve my site. I encountered this problem making vertical rhythm. :slight_smile:

Maybe it’s time to review again Meyer’s book on CSS. :slight_smile:

sorry for such dumb question

first p. is its the box model having 22.5px. put line-height:0 for the elements in it. now computed box model for both p has 21px as height.

i guess i have to break it down my self to RRH.

try and put more content in those paragraphs, or resize your browser window to something little enough that their content will extend over one line. you’ll see that your 21px is not holding, meaning that they are equal only when they have the same number of content lines, in case that the equal size p is what your looking for here. those p will be proportional though, due to the same line height value, given the solution above. also, try and put height: 21px for them to test what happens now when their content extends more than one line.

Yep. :shifty:

sure it’s respected and that’s why you have overflow which is the cause for some nasty looking renderings when you don’t take account of the fact that setting a height might cause the overflow… :lol: (: :eye: :goof:

If you set a height then that height will be respected with block elements.
Content will hang out though

setting a height is not enough to solve all your problems. it can mean more problems too. you need to think about overflow: what happens when the content of an element doesn’t fit within the boundaries of its box model.

Not a dumb question at all, a lot of people have trouble with the inline box model. Even Eric Meyer. It’s by far the most complicated subject in CSS :slight_smile:

Paul O’B: WOW! I’m much impressed how professional feedback I got!
THANK YOU buddies! :slight_smile:

The inline box model is a very complicated thing and a thing of beauty (not).:slight_smile:

Line-height sets the minmum height of the line so it can change depending on its content. You set the line-height to 21px which means that the half leading that is distributed between top and bottom of the line is 21px - 14(font-size) = 7px to be equally distributed between top and bottom = 3.5px each side (ignoring the fact that there is no such thing as half a pixel so there will be some differences in the way that this is distributed between browsers anyway.

Then you have another inline box that has font-size of only 10px which means that 11px will be distributed over the top and bottom = 5.5px each side. However the smaller text sits on the baseline and when the 5.5px half leading is added to the bottom it pushes the line height higher than it was.

You can test this by changing the vertical alignment of the inner element to top and the line reverts to 21px.


<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "">
<html xmlns="">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
p {
    line-height: 21px;
span {
    color: #98B5F2;
    font-size: 10px;


<p>This is some ext <span>in a span</span></p>

(Note that “top” is referring to the text next to it and not the top of the line. This just moves it up so that its half leading doesn’t go outside the existing line-box.)

Therefore when you have inline boxes of varying font-size you should reduce the line height on the smaller font sizes to avoid this happening.

Or as Eric Meyer suggests unitless line-heights would achieve this more simply.

p {
[B]    line-height: 1.5;[/B]
.comment-data {
    color: #98B5F2;

The child elements inherits the scaling factor (because of the unitless line-height) and not the computed height as per other methods and thus keeps the text within the boundaries of the line.

Easy once you know how isn’t it :slight_smile:

I’ve included an attachment as it so much easier to see in a drawing.

Just because you set a line-height doesn’t mean that that is the height of it :). The height will be whatever the calculated height is because you didn’t set a height :slight_smile:

Thanks man a lot! But I still don’t understand this behaviour. Children of P should have been inside P box model (21px) and don’t exceed it. Can you explain it to me? :slight_smile:

simple. some parent elements extend their box model to accommodate their children. that’s the computed box model for you as opposed to the theoretical one.

and you’re welcome :slight_smile: you should practice using developer tools browsers have to offer. firebug is really good.