The font-size CSS property specifies the size of the font . Setting the font size may change the size of other items, since it is used to compute the value of the em and ex <length> units.
/* <absolute-size> values */ font-size: xx-small; font-size: x-small; font-size: small; font-size: medium; font-size: large; font-size: x-large; font-size: xx-large; /* <relative-size> values */ font-size: smaller; font-size: larger; /* <length> values */ font-size: 12px; font-size: 0.8em; /* <percentage> values */ font-size: 80%; /* Global values */ font-size: inherit; font-size: initial; font-size: unset;
| Initial value | medium |
|---|---|
| Applies to | all elements. It also applies to ::first-letter and ::first-line. |
| Inherited | yes |
| Percentages | refer to the parent element's font size |
| Media | visual |
| Computed value | as specified, but with relative lengths converted into absolute lengths |
| Animation type | a length |
| Canonical order | the unique non-ambiguous order defined by the formal grammar |
The font-size property is specified in one of two ways:
<length-percentage> value.xx-small, x-small, small, medium, large, x-large, xx-largemedium). Similar to presentational HTML's <font size="1"> through <font size="7"> where the user's default font size is <font size="3">.larger, smaller<length-percentage>A positive <length> or <percentage> value. When the units of <length> values are specified in em or ex, the size is defined relative to the size of the font on the parent element of the element in question. For example, 0.5em is half the font size of the parent of the current element. When the units are specified in rem, the size is defined relative to the size of the font used by the <html> (root) element.
<percentage>Values refer to the parent element's font size.
It is best to use values that are relative to the user's default font size, and avoid absolute values such as lengths with units other than em or ex. However, if such absolute values must be used, px are preferred over other units because their meaning does not vary depending on what the operating system thinks (generally incorrectly) the resolution of the monitor is.
<absolute-size> | <relative-size> | <length-percentage>where
<absolute-size> = xx-small | x-small | small | medium | large | x-large | xx-large
<relative-size> = larger | smaller
<length-percentage> = <length> | <percentage>
There are several ways to specify the font size, with keywords or numerical values for pixels or ems. Choose the appropriate method based on the needs for the particular web page.
Keywords are a good way to set the size of fonts on the web. By setting a keyword font size on the body element, you can set relative font-sizing everywhere else on the page, giving you the ability to easily scale the font up or down on the entire page accordingly.
Setting the font size in pixel values (px) is a good choice when you need pixel accuracy. A px value is static. This is an OS-independent and cross-browser way of literally telling the browsers to render the letters at exactly the number of pixels in height that you specified. The results may vary slightly across browsers, as they may use different algorithms to achieve a similar effect.
Font sizing settings can also be used in combination. For example, if a parent element is set to 16px and its child element is set to larger, the child element displays larger than the parent element in the page.
Another way of setting the font size is with em values. The size of an em value is dynamic. When defining the font-size property, an em is equal to the size of the font that applies to the parent of the element in question. If you haven't set the font size anywhere on the page, then it is the browser default, which is often 16px. So, by default 1em = 16px, and 2em = 32px. If you set a font-size of 20px on the body element, then 1em = 20px and 2em = 40px. Note that the value 2 is essentially a multiplier of the current em size.
In order to calculate the em equivalent for any pixel value required, you can use this formula:
em = desired element pixel value / parent element font-size in pixels
For example, suppose the font-size of the body of the page is set to 16px. If the font-size you want is 12px, then you should specify 0.75em (because 12/16 = 0.75). Similarly, if you want a font size of 10px, then specify 0.625em (10/16 = 0.625); for 22px, specify 1.375em (22/16).
The em is a very useful unit in CSS, since it automatically adapts its length relative to the font that the reader chooses to use.
One important fact to keep in mind: em values compound. Take the following HTML and CSS:
html {
font-size: 62.5%; /* font-size 1em = 10px on default browser settings */
}
span {
font-size: 1.6em;
} <div> <span>Outer <span>inner</span> outer</span> </div>
The result is:
Assuming that the browser's default font-size is 16px, the words “outer” would be rendered at 16px, but the word “inner” would be rendered at 25.6px. This is because the inner span's font-size is 1.6 em which is relative to its parent's font-size, which is in turn relative to its parent's font-size. This is often called compounding.
rem values were invented in order to sidestep the compounding problem. rem values are relative to the root html element, not the parent element. In other words, it lets you specify a font size in a relative fashion without being affected by the size of the parent, thereby eliminating compounding.
The CSS below is nearly identical to the previous example. The only exception is that the unit has been changed to rem.
html {
font-size: 62.5%; /* font-size 1em = 10px on default browser settings */
}
span {
font-size: 1.6rem;
}
Then we apply this CSS to the same HTML, which looks like this:
<span>Outer <span>inner</span> outer</span>
In this example, the words “outer inner outer” are all displayed at 16px (assuming that the browser's font-size has been left at the default value of 16px).
/* Set paragraph text to be very large. */
p { font-size: xx-large }
/* Set h1 (level 1 heading) text to be 2.5 times the size
* of the text around it. */
h1 { font-size: 250% }
/* Sets text enclosed within span tag to be 16px */
span { font-size: 16px; }
.small {
font-size: xx-small;
}
.larger {
font-size: larger;
}
.point {
font-size: 24pt;
}
.percent {
font-size: 200%;
}
<h1 class="small">Small H1</h1> <h1 class="larger">Larger H1</h1> <h1 class="point">24 point H1</h1> <h1 class="percent">200% H1</h1>
em and ex units on the font-size property are relative to the parent element's font size (unlike all other properties, where they're relative to the font size on the element). This means em units and percentages do the same thing for font-size.
| Specification | Status | Comment |
|---|---|---|
| CSS Fonts Module Level 3 The definition of 'font-size' in that specification. | Candidate Recommendation | No change |
| CSS Transitions The definition of 'font-size' in that specification. | Working Draft | Defines font-size as animatable. |
| CSS Level 2 (Revision 1) The definition of 'font-size' in that specification. | Recommendation | No change |
| CSS Level 1 The definition of 'font-size' in that specification. | Recommendation | Initial definition |
| Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|---|
| Basic support | 1 | 12 | 1 | 5.5 | 7 | 1 |
| Rem values | 31 | 12 | 311 2 |
11 9 — 10 |
28 | 7 |
| Feature | Android webview | Chrome for Android | Edge mobile | Firefox for Android | IE mobile | Opera Android | iOS Safari |
|---|---|---|---|---|---|---|---|
| Basic support | 1 | 1 | 12 | 4 | 6 | 6 | 1 |
| Rem values | 4.1 | 42 | 12 | 31 | 10 | 12 | Yes |
1. Before Firefox 57, animations using em units are not affected by changes to the font-size of the animated element's parent (bug 1254424).
2. Before Firefox 57, some language settings' inherited font-size is smaller than expected (bug 1391341).
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https://developer.mozilla.org/en-US/docs/Web/CSS/font-size