CSS Units Best Practices

css-units-best-practices.md

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;
  }
}

Thank you for this very nice overview!

About "Media Queries in em only" - that seems to be more of a myth than a fact or an actual recommendation.

Way back there were zoom problems in browsers, but in the 2012 article you cite the author has added this to the article in 2015:

You should note that the zooming behavior has long since been made consistent in browsers (i.e. fixed). Keep that in mind if you cite or otherwise put stock in this post. I never meant to be absolutely prescriptive about ems as units—use whatever feels natural and appropriate for your design strategy.

In other words: Media Queries in px are no problem.

Also there is a question about this from Stephen Hay to Ethan Marcotte right at the beginning of the "Speaker Panel | CSS Day 2014": https://vimeo.com/102847187#t=30s

Ethan prefers MQs in em but thinks that "it's more stylistic" and that "pixels are just fine".

Thanks for the post, and thanks @pmmueller for adding your note about this!

Why not use ch and ex frequently? They seem perfect for width and height

Great post.. Thanks

Excuse the dumb question but what do you mean by Print?

Excuse the dumb question but what do you mean by Print?

@marwanbayoumi You can use CSS to style a page differently whenever it is printed. In such cases, using a physical unit may be more appropriate. See these resources for more on this topic:

1. A Guide To The State Of Print Stylesheets In 2018 (Smashing Magazine)
2. Printing (MDN Web Docs)

Why not use ch and ex frequently? They seem perfect for width and height

@bn-l This guide is a bit bias toward web apps. When wanting to maintain fixed sizing and spacing throughout a user interface, it may be better to primarily use rem, as that provides a way to be flexible and consistent.

In the case of content sites, ch and ex units may occasionally be more appropriate. For example, if wanting to stay within the recommended 45–90 characters per line to improve readability, you could do this:

p {
 width: 45ch;
}

To calculate a similar width with another unit would involve math or trial-and-error. In this case, ch is a very straightforward solution.

@basham Practices about scaling the elements for high DPI screens like smartphone screens would be good. Really the use o fixed pixels becomes a mess when the site is displayed in a screen with different DPIs, which always happen when the website is public.

@basham Practices about scaling the elements for high DPI screens like smartphone screens would be good. Really the use o fixed pixels becomes a mess when the site is displayed in a screen with different DPIs, which always happen when the website is public.

@HMaker Do you have any recommendations in this area or have any suggestions for such a resource? I haven't studied this topic. Don't know if there's too much more than a resolution media query.

"Only use em within media query definitions, never pixels." - the article from 2012. I think this has been fixed or am I wrong?

Isn't it more responsive if we use rem for font-size rather than em.?

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.

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

That's what I've been looking for 😃

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

Verify Github on Galxe. gid:mGErGTSTvcAM2BuipcSWyn

nice/ but this wont work perfectly for media queries

vw/vh should be updated with lvh, lvw, dvw, dvh and the co.

Shouldn't we use 'rem' than 'em'?