Unit for sizes in CSS

Question

The last days I dealt with units in responsive webdesign and webdesign in general. My question is about units for sizes in CSS at all, meaning not only related to font-size, but also padding, width, media queries, ...

It seems as px is considered pretty for most use cases bad by most professional web designers.

Everyone against px

Some articles that let me come to this assumption

Here an article that has also an opinion pro px: Should I use px or rem value units in my CSS?

Reasons against px / for (r)em?

media queries

The big advantage of em media queries is, following many articles, that these media queries are even applied when using the browser's text-only zoom.

However I think all modern browsers support full-page zoom, which also applies media queries defined in pixels (tested in up-to-date Firefox and IE).

accessibility

em (or rem) seems to be recommended for font-size (accessibility) but what is about the other sizes?

Increased accessibility is IMHO no important reason anymore, because modern browsers support full-page zoom. I have no idea about screenreaders, but I can't imagine, that they haven't been improved in the last years.

maintainability

It is easy to change the size of the complete site by changing the base size.

If it is a persistent change for every device it should be very easy when using a CSS preprocessor.

The only reason I could find was in the css-tricks.com article, because you may want to have all sizes changed on mobile devices. Do you really want this for all / most elements? For some cases like really big headings it seems to be useful, but why should a mobile user be able to read smaller text better than a desktop user? Additionally the measuring in reference pixels compensates the different DPIs.

Real life examples

I have roughly analyzed their source code, probably I have overlooked some things.

Bootstrap 3

In most places px is used: font-size, media queries, padding, margin ... The grid system uses percents at least for widths. However you can find many issues regarding this topic at Github. Many people demand em / rem.

ZURB Foundation 5

It is based on rem and em and in little cases it uses px, but just for borders etc.

cloudfour.com

Media queries and font-size in em and a mix of different units for everything else: px, %, em.

css-tricks.com

font-size, padding, margin mostly in px, but media queries in em and some widths in %.

So far I have read everywhere, that you should use relative sizes and have done so. If there are sensible reasons for (and I thinks so, because many professionals do so) I will continue / improve doing so, but it was more complicated than using px.

If you assume most people are using full-page zoom, where is the problem with px for everything, having one consistent unit seems to be nice? (Supporting people, who haven't upgraded their browser / device for almost 10 years (IE <= 7), is not my aim.)

Which is the way to go? (Please consider the different properties font-size, width, media queries, ...)

Usually here on stackoverflow I can find pretty many detailled answers, is stackoverflow the wrong place for this kind of question or why is there just 1 answer? — Niklas Peter, Mar 09 '14 at 13:16
looks like you got some good answers now -- want to mark one of them? — sandover, Mar 28 '14 at 19:22
Done. I just wanted to wait until I have finished refactoring. — Niklas Peter, Mar 28 '14 at 20:40

score 9 · Accepted Answer · edited May 23 '17 at 12:17

TL;DR:

Use % for horizontal page layout
em works great for vertical spacing
Use anything you prefer for font-size (I like em)
Use em for media queries

CSS Measurement Units

I find that my usage of different CSS measurement units varies with the area of measurement I'm dealing with: horizontal, vertical, typography, or media query.

Horizontal measurements

The horizontal space gives structure to the page. Thus, horizontal measurements work well with units relative to the page width (%). For example, it's great for giving some basic breathing room to the main content:

#main {
  width: 90%;
  margin: 0 auto;
}

More complex layouts (columns, for example) also work well when one or all of the horizontal measurements are proportional to the page or to their container.

.column-main {
  width: 61.8%; /* The golden ratio is pretty awesome. */
}

.column-secondary {
  width: 38.2%;
}

There's a corollary to this idea, which is that if you set up your container structure well, you won't have to do much horizontal sizing on the contents. You basically let the block-level elements fill the width of their container, as they like to do, and you're all set.

Vertical measurements

Vertical space is more about the structure of the content (i.e. how close elements in the document flow are to one another), and I think these measurements work fine with both fixed and relative units (px or em).

However, using relative units here helps give you a vertical rhythm, which can be very pleasing. It also allows you to make changes to your content (read: font-size) and keep your vertical spacing proportional.

p, ul, ol, dl {
  margin: 0 1em; /* gives you proportional spacing above and below, 
                    no matter the font size */
}

Typographic measurements

Font measurements fall into their own category, maybe because font-size acts as the base for all other measurements in em. I've seen advocates for different strategies, but from what I've seen any measurement will work fine, as long as you know what you're doing.

px

Setting font-size in px is extremely reliable and easy to calculate. In the post-IE6 age, it also scales nicely, so there's really no reason not to use px if that's what you prefer.

The only problem I see in using px is that it doesn't take advantage of the CSS cascade. In other words, once I've specified sizes in px, I have to go back and change each one of them individually if I want to make any large-scale changes.

em

Despite any drawbacks, I think em can be a really good way to specify font-size. What I like is that it lets me quickly see the relationship between my font sizes. A lot of times I don't care what the exact size of the font is because the most important thing for me is the relationship of that size to all of the other sizes on the page.

The important thing about using em is to set the sizes as close to the end tag as possible. That means I avoid setting font units on containers:

aside { font-size: 0.8em; } /* This can mess me up */
...
aside p { font-size: 0.8em; } /* Way too small! */

And mostly set sizes on headings and text items:

h1 { font-size: 2.5em; }
h2 { font-size: 2.1em; }
h3 { font-size: 1.7em; }
...

Anyway, I like that I can see clearly and easily the relationship between the sizes there.

rem

Sizing things based on the browser's base font size seems like the web-standards thing to do, because then you allow for browsers whose optimal base font size might not be 16px. In practice, though, it kind of works the other way. From what I've seen, browsers use 16px as the base font size because that's what everyone expects, and set the actual size of that CSS measurement to look decent in the browser.

The biggest drawback here I see is browser support. If you code for browsers that don't support rem, you'll be adding your rules twice:

p {
  font-size: 16px;
  font-size: 1rem; /* I kind of hate repeating myself here,
                      but a good CSS preprocessor might ease that pain. */
}

Font Spacing

Other font measurements are a lot easier to figure out once you've got your font-size since most other font measurements fall into the category of either vertical or horizontal measure. For example:

h1 {
  font-size: 2.5em; /* Change this, and everything below stays proportional.
                       (Use whatever measurement unit you prefer.) */
  margin-top: 0.618em;
  margin-bottom: 0.3em;
  line-height: 1.2; /* The only unit-less measure in CSS */
}

Media queries

In most cases, there is little difference between the way browsers handle em and px in media queries. This holds true even when a user uses text zoom to resize the page.

HOWEVER, if someone has changed the default text size in their browser settings, things behave very differently. See my answer to a related question and my codepen demo for a longer explanation.

In short, while px works well for media queries in most cases, it's definitely safer to go with em.

Other cases

There are certainly cases where the above general comments don't apply. For example, you may find that ems come in handy any time you want things proportional to the font size, like the following:

h1.title {
  font-size: 2em;
  width: 20em; /* Resizing the font also resizes its container properly. */
  background-color: #d00d1e;
}

@media (min-width: 400px) {
  h1 {
    font-size: 2.5em; /* Woot! Didn't have to touch the width! */
  }
}

Or maybe you want to constrain line lengths for readability:

p {
  max-width: 42em;
}

Also, as mentioned in other comments, px still works well for borders and things like that. It can also be good for padding, especially when you pair it with box-sizing:

.example {
  width: 80%;
  box-sizing: border-box; /* Padding is NOT added to total width. Huzzah! */
  padding: 10px;
}

score 1 · Answer 2 · edited May 23 '17 at 12:32

There is already a great answer for this question: Should I use PX or REM?

But to add to that, the articles you link to for "All against px" are generally based on misunderstandings or mis-assumptions.

For example, PX are relative (see the other answer). From an accessibility point of view (my area of exertise), zoom is now the default method of making things bigger/smaller, so PX vs EM is irrelevant. (The browser renders to CSS pixels in both cases.)

Also (for the Cloudfour article) Webkit used to have a bug, but it fixed that a while ago (pre split with blink) so that PX now trigger media queries on zoom.

Basically, whatever you use is translated to (CSS) pixels anyway, so it is up to you.

Personally, I set a based size in PX (generally 16px) and then size in REM. That means it is easy to set the sizes relative to the base, including padding/margin etc. Then you can easily change the base pixels within different media queries. You can also provide PX fallbacks for older browsers if needed.

score 0 · Answer 3 · answered Mar 05 '14 at 17:00

I have personally made the switch EMs and haven't had any issues with it. It works great for anything you'd normally put pixels in. There are only a few scenarios where I use px still and that is for borders and shadows and other things I don't want growing/shrinking with the font size.

Since EMs are somewhat of a dynamically calculated PX size they work well in the situations where you "know" your size. A great tool for converting PX to EM (or vice versa)

http://pxtoem.com/

I do use % for fluid elements rather than define a specific grid system which has worked well for my own needs. But I haven't tried that approach while also using third-party grid/responsive frameworks. So mileage may vary there.