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CSS Units Best Practices

CSS units

Recommendations of unit types per media type:

Media Recommended Occasional use Infrequent use Not recommended
Screen em, rem, % px ch, ex, vw, vh, vmin, vmax cm, mm, in, pt, pc
Print em, rem, % cm, mm, in, pt, pc ch, ex px, vw, vh, vmin, vmax

Relative units

Relative units play nicely with both screen and print media types and should be the most frequently used CSS units.

Unit Description
% percentage, relative to the same property of the parent element
em relative to font size of the element
rem relative to font size of the root element
ch relative to width of the "0" (ZERO, U+0030) glyph in the element's font
ex relative to x-height of the font

Absolute units

Physical units (e.g. cm, mm, in, pc, and pt) should only be used for print style sheets, while pixels (px) should only be used for the screen. While there are consistent conversions among all of these absolute length units, depending on the device, CSS units can actually mean different things. For example, while 1cm in CSS should print as one physical centimeter, there's no guarantee that 1cm in CSS results in one physical centimeter on the screen.

Unit Description cm mm in pc pt px
cm centimeter 1cm 10mm
mm millimeter 1/10cm 1mm
in inch 2.54cm 25.4mm 1in 6pc 72pt 96px
pc pica 1/6in 1pc 12pt 16px
pt point 1/72in 1/12pc 1pt 4/3px
px pixel 1/96in 1/16pc 3/4pt 1px

Viewport units

Viewport-percentage length units should be used with caution, as there is still some lingering bugs with their implementation.

Unit Description
vw relative to 1% of viewport's width
vh relative to 1% of viewport's height
vmin relative to 1% of viewport's smaller dimension
vmax relative to 1% of viewport's larger dimension

Contexts

Document-level

Assume the root font size is 16px but don't require it to be so. Either declare the font size as 100% or don't declare the font-size property at all.

html {
  font-size: 100%;
}

Borders

Most likely use px, as most of the time, the border shouldn't need to scale.

.Component {
  border-bottom: 1px solid #ccc;
}

Font-size

Font-size should only be applied at the lowest possible child elements, in order to minimize its cascading impact on relative units. While both of the following two examples are essentially equivalent, only the first is recommended.

DO:

.Component {
}
.Component-heading {
  font-size: 1.2em;
}
.Component-body {
  font-size: 0.9em;
}

DO NOT:

.Component {
  font-size: 1.2em;
}
.Component-heading {
  font-size: 1em;
}
.Component-body {
  font-size: 0.75em;
}

Padding and margin

In order to ensure consistent use of whitespace throughout the application, given a component could be used in a variety of contexts, it may be best to use rem for margin and padding than em.

.Component {
  margin: 1rem 0;
  padding: 1rem;
}

Media queries

Only use em within media query definitions, never pixels. Until there's wider rem support within media queries, rem should be avoided in media queries as well.

@media (min-width: 20em) {
  .Component {
    background-color: blue;
  }
}
@akramnarejo
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Isn't it more responsive if we use rem for font-size rather than em.?

@berzi
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berzi commented Sep 16, 2021

This document should be updated, it shows up quite high on Google but contains many outdated concepts, for example based on limited feature support, even when caniuse now shows very good support (see for example vmin/vmax or rem on "usare relative" view, which clearly shows unsupported browser versions are almost completely unused at this point).

The vmin/vmax section in particular could use some expansion on the use case, and the em section should probably recommend the use of rem, which doesn't yield the same issues as em, since it doesn't inherit from parent elements in cascade.

Furthermore, the use of percentages is promoted as though it would solve any problem, but that is not the case and it's important to redirect the user. See for example this question on SE.

@andrei9669
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@berzi there's also this article that suggests not to use relative units on layout. altho, I don't know how relevant it is.

@faisal95bd
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That's what I've been looking for 😃

@vikramvi
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For media queries do we need to still use em vs rem ?

@allaboutnft
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Verify Github on Galxe. gid:mGErGTSTvcAM2BuipcSWyn

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