How can I override the minimum-font-size in Firefox?

Question

My site displays just like in need in IE and Opera, but in Firefox I can't get (via CSS) to have font sizes smaller than Firefox' default minimum-font-size. Of course I can go to Tools>Options and remove this value, but other users will see some of the elements displaced.

I have tried using !important, but it doesn't work. And I can't substitute the text by images, they are dynamic fields.

Answer 1

Okay. I'm going to explain how you do this, but you need to know a few things first.

1. You shouldn't be doing this.

There is a reason, and a very good one, that the minimum font size setting exists. Simply, with most fonts, it is already a struggle to read anything below 12px, never mind below the default minimum of 9px. If your design calls for such font sizes for anything but an extremely restricted set of circumstances¹, then your design is just bad and excludes a huge category of users.

2. You really shouldn't be doing this.

The web is an intrinsically flexible medium, and a good design must take account of this. Designs that require a fixed font size to work are suitable for print, but they are not suitable for the web. Any designer who cannot work with this requirement does not understand the medium, and no decent PM should prioritise a designer's bad decisions over practicality and usability. You have far bigger, more fundamental problems to deal with if you can't argue against this decision.

3. You really, really shouldn't be doing this.

Depending on your jurisdiction, you may be wandering on a legal grey area. Tiny font sizes are difficult to read, and will make it very easy for your users to miss information—which is especially troublesome if the text at hand is legally significant, such as opt-out information or a disclaimer. You are also treading on thin ice with respect to the accessibility of your site by causing serious problems for people with visual impairments.

How to do it anyway

It is quite easy to make Firefox display fonts at less than the minimum specified size: just use the font-size-adjust property.

Here comes the science bit

Every font has something called an aspect value, which is the ratio between its x-height (the height of a letter x)² and the actual font-size you set. In Comic Sans, for example, this is really big (0.55), as you can tell by how big the shorter letters (a, c, e…) are compared to taller letters (b, d, h…), whereas in Courier New it is a lot smaller (0.43).

This variability can cause some problems. For example, let's say you specify a super-fancy font for your body text, but because you're a good designer you provide several fallback fonts. That's great, but because some of those fallbacks have different aspect values, letters might be smaller and less legible at the size you specified. font-size-adjust allows you to make sure that any fallback has the same x-height as your super-fancy font.

Unfortunately, we can also use it to make text less legible. Let's say your body copy is Segoe UI at 12px:

body {
    font-family: "Segoe UI";
    font-size: 12px;
}

The aspect value of Segoe UI is almost exactly 0.5, so if you specify that as the font-size-adjust value, your font size doesn't change:

body {
    font-family: "Segoe UI";
    font-size: 12px;
    font-size-adjust: 0.5; /* x-height = 12px × 0.5 = 6px */
}

What happens if we set it to something smaller? Well, the browser will adjust the font size to use an x-height equal to font-size × font-size-adjust. The below, for example, will result in an effective font-size of 6px:

body {
    font-family: "Segoe UI";
    font-size: 12px;
    font-size-adjust: 0.25; /* x-height = 12px × 0.25 = 3px */
}

This forms the basis of our attack. It turns out that Firefox will honour font-size-adjust even if it overrides the minimum font size. (It goes without saying that this is a bug.)

The demo

Try this out in Firefox with a minimum font-size set. It exploits two bugs: the aforementioned font-size-adjust issue and a separate issue with reporting the rendered font size via getComputedStyle.

You need to know the aspect value of the font you are using for this to calculate the right font-size-adjust value, for which this list of aspect values might help you. (Alternatively, try an estimated x-height for fonts installed on your system.)

But wait! There's worse!

The above technique only works in Firefox. It's even easier to override the minimum font size on an element in Webkit, including Safari, iOS Safari and Chrome—just use the non-standard -webkit-text-size-adjust property:

-webkit-text-size-adjust: none;

This is another bug, by the way. text-size-adjust is a non-standard proposal intended to limit the extent to which text-size is automatically inflated on mobile devices. It isn't meant to work on desktop UAs, much less prevent manual text resizing.

Last word: it won't last forever

Bugs get fixed eventually. You are not meant to be able to override the browser's default font size, no matter how much your irresponsible designer and PM want you to. Sooner or later, people will figure out that these properties are being exploited to make the web worse for everybody, someone will land a patch and the fun will be over.

That said, if you are being forced into this, I wholeheartedly advocate a solution that will eventually force the parties involved to rethink their decision.

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¹ Hint: if your text conveys any information that is meant to be readable by anyone, ever, then you shouldn't be messing with browser accessibility settings. Legitimate cases include purely decorative effects (complex ASCII art, shape poetry) and document previews.

² More correctly, the distance from the baseline to the median height, which is the height of a letter x in most fonts.

Answer 2

You cannot do it. It's a setting that Firefox provides so that people like us cannot override it with some CSS settings.