font-size vs line-height vs actual height - css

Question

This question's answer says that the font-size defines the height of a box so that all letters (with ascenders and descenders) can fit.
But why has a span with 40px font-size and line-height an actual size of 45px? If I understand the linked question correctly then "X" should be smaller than 40px but the overall height should be exactly 40px. I thought that maybe it is making some extra room above/below the ascenders/descenders but the image shows that the ascenders/descenders take all the space so there can't be much extra room:
When I wrap a div (green) around the span then the div has a height of 40px. Why does the div use the font-size of its child for its height but the child itself doesn't?:
Now when I set the span's line-height to 15px (less than the font-size) then the div's height changes to 26px. How is this value calculated? Has this something to do with the baseline?:
When I set the span's line-height to 65px (more than the font-size) then the div's height is the height of the line-height. I would have expected the div's height to be something like (65px - 45px) + 45px.:
So how do font-size and line-height affect the actual heights of elements? I read some questions that referenced the spec but I couldn't make much out of it. Are there any easy to understand rules?
JSFiddle

Answer 1

First, I recommend reading my answer in Inline elements and line-height. To summarize, there are various heights related to inline boxes:
Height of the inline box, given by line-height
Height of the line box, which in simple cases is also given by line-height, but not here.
Height of the content area of the inline box, which is implementation dependent. This is the area painted by the red background.
The other height in your case is the height of the parent div. This is determined by §10.6.3. In this case, since the box establishes an inline formatting context with one line,
The element's height is the distance from its top content edge to [...] the bottom edge of the last line box
So the height of the parent block is given by the height of the line box.
What happens here is that the height of the line box is not the line-height of your inline box. And that's because the line-height of the parent block is also taken into account:
On a block container element whose content is composed of inline-level elements, 'line-height' specifies the minimal height of line boxes within the element.
The minimum height consists of a minimum height
above the baseline and a minimum depth below it, exactly as if each
line box starts with a zero-width inline box with the element's font
and line height properties.
We call that imaginary box a "strut."
If you set parent's line-height to 0, and child's vertical-align to e.g top, then the height of the parent will exactly be the line-height of the child.
.outer {
margin-top: 50px;
background-color: green;
width: 150px;
font-family: "Times New Roman";
line-height: 0;
}
.letter-span-1 {
background-color: red;
line-height: 40px;
font-size: 40px;
vertical-align: top;
}
.letter-span-2 {
background-color: red;
line-height: 15px;
font-size: 40px;
vertical-align: top;
}
.letter-span-3 {
background-color: red;
line-height: 65px;
font-size: 40px;
vertical-align: top;
}
<span class="letter-span-1">XxÀg</span>
<div class="outer">
<span class="letter-span-1">XxÀg</span>
</div>
The parent block is 40px tall.
<div class="outer">
<span class="letter-span-2">XxAg</span>
</div>
The parent block is 15px tall.
<div class="outer">
<span class="letter-span-3">XxÀg</span>
</div>
The parent block is 65px tall.
If you don't set a line-height to the parent, it will be normal.
Tells user agents to set the used value to a "reasonable" value based
on the font of the element[...]. We recommend a used value for
'normal' between 1.0 to 1.2.
That means that there will be a minimum height for the parent which will be its font-size (which you don't specify, and the default is implementation-dependent) multiplied by that factor (implementation-dependent).
You should also consider the vertical-align of the span. By default it's baseline, and that may create a gap below. The image in web-tiki's answer is especially useful:
That's because vertical-align determines how the span will be aligned with the strut, and with baseline the alignment can depend on font-size and end up increasing the height of the line box. The line box height is the distance between the top of the uppermost and the bottom of the lowermost boxes in the line.
If you don't want the height of the parent div to be increased by that, you need some other vertical-align, like top, middle, or bottom. Then the font-size of the span shouldn't affect the height of the div.
To summarize, the height of the div depends on
Its line-height
... which by default depends on div's font-size
Span's line-height
... which by default depends on span's font-size
Possibly span's font-size, depending on span's vertical-align
And obviously height, min-height, max-height, etc.

Answer 2

Introduction
Good question,
I learn most of these things through personal experience.
In this case, the DIV height is set to auto. It will detect the height of its own contents, and evaluate its new height from there.
Clearly, the DIV only takes into account the line height of the . This is likely due to the diverse number of fonts. Line-height gives us the adaptability we need for various font types.
In Short
font-size
Font size only changes the actual font itself, and not the div elements around it
line-height
Line-height is the height of the actual line and will change the div elements around it
Wait a second...
If we have something like this:
div {
background: green;
margin-top: 50px;
}
.test-one {
font-size: 20px
}
.test-two {
font-size: 40px
}
<div>
<span class="test-one"> test one </span>
</div>
<div>
<span class="test-two"> test one </span>
</div>
Clearly the size of the DIV (height: auto;) changes according to font-size. That's because the line-height will automatically adjust accordingly if it is not set manually.
One Exception
Upon further inspection, I noticed that DIVs don't always match the line-height. This occurs if the line-height is very small, and the font exceeds it by some distance.
The example you gave -
.outer {
margin-top: 50px;
background-color: green;
width: 150px;
font-family: "Times New Roman"
}
.letter-span-1 {
background-color: red;
line-height: 40px;
font-size: 40px;
}
.letter-span-2 {
background-color: red;
line-height: 15px;
font-size: 40px;
}
.letter-span-3 {
background-color: red;
line-height: 65px;
font-size: 40px;
}
<span class="letter-span-1">XxÀg</span>
<div class="outer">
<span class="letter-span-1">XxÀg</span>
</div>
<div class="outer">
<span class="letter-span-2">XxÀg</span>
</div>
<div class="outer">
<span class="letter-span-3">XxÀg</span>
</div>
If you look closely,
letter-span-1 and letter-span-3 both result in the DIV equaling the line-height.
However, letter-span-2 does not.
-------------- Line-height - Actual-height
letter-span-1: 40px - 40px
letter-span-2: 15px - 25px
letter-span-3: 65px - 65px
Notice that letter-span-2 is the smallest. It is so small, it will actually limit the height of the div. You can test this by altering the font size.
The "Why?"
Why have these two different settings, and not just change height normally?
I honestly am not sure, but I speculate that it was because fonts aren't standard. If the computer misreads a particular font, it may incorrectly evaluate the line-height.
Not to mention the numerous "CSS Tricks" you can do with line-height. It is great for adding space for open designs.
Conclusion
Line-height defines div height, unless line-height is very small, in which case the font-size will define the size.