Why is font-size unreliable when universally applied?

Issue

With the following CSS:

* {
    margin: 0;
    padding: 0;
    font-family:"Segoe UI";
    font-variant: small-caps;
}
.red {
    color: hsl(0, 100%, 50%);
    font-size: 3em;
}
.orange {
    color: hsl(30, 100%, 50%);
    font-size: 3em;
}
.yellow {
    color: hsl(60, 100%, 50%);
    font-size: 3em;
}
.green {
    color: hsl(120, 100%, 50%);
    font-size: 3em;
}
.blue {
    color: hsl(210, 100%, 50%);
    font-size: 3em;
}
.indigo {
    color: hsl(230, 100%, 50%);
    font-size: 3em;
}
.violet {
    color: hsl(274, 100%, 50%);
    font-size: 3em;
}

...the attributes in the * selector work fine - they apply to all of the classes; however, if I move font-size there, the text grows to tyrannosauric proportions. You can see this by moving the font-size from the individual classes to the * class here: http://jsfiddle.net/NvTvr/8/

Why does this happen?

UPDATE

So this is the way to go, then:

* {
    margin: 0;
    padding: 0;
    font-family:"Segoe UI";
    font-variant: small-caps;
    font-size: 48px;
}
.red {
    color: hsl(0, 100%, 50%);
}

...etc. (no font-size specified within the color classes); as seen in http://jsfiddle.net/NvTvr/10/

Solution

Because em is relative to the context.

As font-size is inherited from the context, each descendant element with em font-size will multiply the current font-size up to the element that contains text content.

Let's see in practice, assuming:

* { font-size: 3em }

You're applying that (relative to context) font-size to all elements.

That means, the html element will have 3 times the default browser font-size. The body will have 3 times font-size of its container (html) which corresponds to 9 times the default font-size. And so forth for each element's ancestors.

There's a new unit that does not have this issue, rem which is relative to root. But this is not as widely supported (no IE<=8 support).

---

Going a bit off-topic for the em x px x rem debate in this section of the answer.

Quoting this answer:

Use em when you specifically want the size of something to depend on the current font size.

Modern browsers can scale px units just fine. em was used mostly when old IE was predominant and didn't scale the px font-size when zooming.

Still, em has it uses in web design. Say, when making CSS-icons that should scale relatively to the font-size of the text. But for overall layout, I go with rem/px which do not generate compound issues and headaches, or percentages when making fluid layouts.

The issue with px for font-size is that, if you ever need to change the overall size of the page's text, you will have to change every single font-size declaration that uses px. That's where rem comes to aid.

There's always an heated discussion about which unit to use for the font-size. Go with the one that gives less headaches and suffices for your project.

So in sum, it depends on personal preference and project requirements.

---

References:

Definitions and explanations

• CSS Length units - MDN
• Why em instead of px? - SO question
• Font sizing with rem - Snook.ca

More discussions

• Should I use px or rem in my css? - SO question (recommends px for font-size)
• Why Ems? - CSS-Tricks (recommends em for font-size)

Answered By - Fabrício Matté