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 |
| em, rem, % | cm, mm, in, pt, pc | ch, ex | px, vw, vh, vmin, vmax |
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 |
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-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 |
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%;
}Most likely use px, as most of the time, the border shouldn't need to scale.
.Component {
border-bottom: 1px solid #ccc;
}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;
}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;
}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;
}
}
@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.