CSS: 100% font size - 100% of what?

Question

There are many articles and questions about percentage-sized vs other-sized fonts. However, I can not find out WHAT the reference of the percent-value is supposed to be. I understand this is 'the same size in all browsers'. I also read this, for instance:

Percent (%): The percent unit is much like the “em” unit, save for a few fundamental differences. First and foremost, the current font-size is equal to 100% (i.e. 12pt = 100%). While using the percent unit, your text remains fully scalable for mobile devices and for accessibility.

Source: http://kyleschaeffer.com/best-practices/css-font-size-em-vs-px-vs-pt-vs/

But if you say "ie 12 pt = 100%", then it means you first have to define font-size: 12pt. Is that how it works? You first define a size in an absolute measure, and then refer to this as '100%'? Does not make a lot of sense, as many samples say it is useful to put:

body {
  font-size: 100%;
}

So by doing this, WHAT is the font size relative to? I notice that the size I see on my screen differs for every font. Arial looks way bigger than Times New Roman, for instance. Also, if I would just do this, body size = 100%, would that mean that it will be the same on all browsers? Or only if I first define an absolute value?

UPDATE, SAT JUL 23

I am getting there, but please bear with me.

So, the % value relates to the default browser font size, if I understand correctly. Well, that is nice but gives me again several other questions:

1. Is this standard size always the same for every browser version, or do they vary between versions?
2. I ! found (see image below) the settings for Google Chrome (never looked at this before!), and I see standard "serif", "sans-serif" and "monospace" settings. But how do I interpret this for other fonts? Say I define font: 100% Georgia;, what size will the browser take? Will it look up the standard size for serif, or has the "Georgia" font a standard size for the browser
3. On several websites I read things like Sizing text and line-height in ems, with a percentage specified on the body [..], was shown to provide **accurate, resizable text across all browsers** in common use today. But from what I am learning now I believe that you should actually choose between either resizable text (using % or em, like what they recommend in this quote), or having 'accurate, consistent font-sizes across browsers' (by using px or pt as a base). Is this correct?

Google Settings:

This is how I think things could look like if you do not define the size in absolute values.

Answer 1

The browser default which is something like 16pt for Firefox, You can check by going into Firefox options, clicking the Content tab, and checking the font size. You can do the same for other browsers as well.

I personally like to control the default font size of my websites, so in a CSS file that is included in every page I will set the BODY default, like so:

body {
    font-family: Helvetica, Arial, sans-serif;
    font-size: 14px
}

Now the font-size of all my HTML tags will inherit a font-size of 14px.

Say that I want a all divs to have a font size 10% bigger than body, I simply do:

div {
    font-size: 110%
}

Now any browser that view my pages will autmoatically make all divs 10% bigger than that of the body, which should be something like 15.4px.

If I want the font-size of all div's to be 10% smaller, I do:

div {
    font-size: 90%
}

This will make all divs have a font-size of 12.6px.

Also you should know that since font-size is inherited, that each nested div will decrease in font size by 10%, so:

<div>Outer DIV.
    <div>Inner DIV</div>
</div>

The inner div will have a font-size of 11.34px (90% of 12.6px), which may not have been intended.

This can help in the explanation: http://www.w3.org/TR/2011/REC-CSS2-20110607/syndata.html#value-def-percentage

Answer 2

My understanding is that when the font is set as follows

body {
  font-size: 100%;
}

the browser will render the font as per the user settings for that browser.

The spec says that % is rendered

relative to parent element's font size

http://www.w3.org/TR/CSS1/#font-size

In this case, I take that to mean what the browser is set to.

Answer 3

A percentage in the value of the font-size property is relative to the parent element’s font size. CSS 2.1 says this obscurely and confusingly (referring to “inherited font size”), but CSS3 Text says it very clearly.

The parent of the body element is the root element, i.e. the html element. Unless set in a style sheet, the font size of the root element is implementation-dependent. It typically depends on user settings.

Setting font-size: 100% is pointless in many cases, as an element inherits its parent’s font size (leading to the same result), if no style sheet sets its own font size. However, it can be useful to override settings in other style sheets (including browser default style sheets).

For example, an input element typically has a setting in browser style sheet, making its default font size smaller than that of copy text. If you wish to make the font size the same, you can set

input { font-size: 100% }

For the body element, the logically redundant setting font-size: 100% is used fairly often, as it is believed to help against some browser bugs (in browsers that probably have lost their significance now).

Answer 4

It's relative to default browser font-size unless you override it with a value in pt or px.

Answer 5

Sorry if I'm late to the party, but in your edit you make a remark about font: 100% Georgia, which the other answers haven't responded to.

There is a difference between font: 100% Georgia and font-size:100%; font-family:'Georgia'. If that was all the shorthand method did, the font-size part would be meaningless. But it also sets a lot of properties to their default values: the line height becomes normal (or around 1.2), ditto for the style (non-italic) and weight (non-bold).

That's all. The other answers already mentioned everything else there was to mention.

Answer 6

As you showed convincingly, the font-size: 100%; will not render the same in all browsers. However, you will set your font face in your CSS file, so this will be the same (or a fallback) in all browsers.

I believe font-size: 100%; can be very useful when combining it with em-based design. As this article shows, this will create a very flexible website.

When is this useful? When your site needs to adapt to the visitors' wishes. Take for example an elderly man that puts his default font-size at 24 px. Or someone with a small screen with a large resolution that increases his default font-size because he otherwise has to squint. Most sites would break, but em-based sites are able to cope with these situations.

Answer 7

Relative to the default size defined to that font.

If someone opens your page on a web browser, there's a default font and font size it uses.

Answer 8

According to ALL THE SPECS DATING BACK TO 1996, percentage values on font-size refer to the parent element's (computed) font-size.

<style>
div {
  font-size: 16px;
}
span {
  font-size: 75%;
}
</style>
<div><span>this font size is 12px!</span></div>

It's that easy.

What if the div declares a relative font-size, like ems, or even worse, another percentage?? See “computed” above. Whatever absolute unit the relative unit converts to.

The only question that remains is what happens when you use a percentage value on the root element, which has no parent:

html {
  font-size: 62.5%; /* 62.5% of what? */
}

In that case, see the “duplicate” of this question. TLDR: percentages on the root element refer to the browser default font size, which might be different per user.

References:

• CSS 1 spec (1996)
• CSS 2.1 spec (2011)
• CSS Fonts Level 3 spec (2013)
• CSS Fonts Level 3 editor’s draft (2017)

Answer 9

As to my understanding it help your content adjust with different values of font family and font sizes.Thus making your content scalable. As to the issue of inhering font size we can always override by giving a specific font size for the element.