CSS - Percentages or Pixels?
Mobile webpages. %'s rock for that. As it will (obviously) adjust to suit.
However when you want a fixed width for say example an article of text that you want displayed in a certain way, px's is the winner.
Both have many plus's and minus's over each other :)
Should I use px or rem value units in my CSS?
TL;DR: use px
.
The Facts
First, it's extremely important to know that per spec, the CSS
px
unit does not equal one physical display pixel. This has always been true – even in the 1996 CSS 1 spec.CSS defines the reference pixel, which measures the size of a pixel on a 96 dpi display. On a display that has a dpi substantially different than 96dpi (like Retina displays), the user agent rescales the
px
unit so that its size matches that of a reference pixel. In other words, this rescaling is exactly why 1 CSS pixel equals 2 physical Retina display pixels.That said, up until 2010 (and the mobile zoom situation notwithstanding), the
px
almost always did equal one physical pixel, because all widely available displays were around 96dpi.Sizes specified in
em
s are relative to the parent element. This leads to theem
's "compounding problem" where nested elements get progressively larger or smaller. For example:body { font-size:20px; }
div { font-size:0.5em; }Gives us:
<body> - 20px
<div> - 10px
<div> - 5px
<div> - 2.5px
<div> - 1.25pxThe CSS3
rem
, which is always relative only to the roothtml
element, is now supported on 99.67% of all browsers in use.
The Opinion
I think everyone agrees that it's good to design your pages to be accommodating to everyone, and to make consideration for the visually impaired. One such consideration (but not the only one!) is allowing users to make the text of your site bigger, so that it's easier to read.
In the beginning, the only way to provide users a way to scale text size was by using relative size units (such as em
s). This is because the browser's font size menu simply changed the root font size. Thus, if you specified font sizes in px
, they wouldn't scale when changing the browser's font size option.
Modern browsers (and even the not-so-modern IE7) all changed the default scaling method to simply zooming in on everything, including images and box sizes. Essentially, they make the reference pixel larger or smaller.
Yes, someone could still change their browser default stylesheet to tweak the default font size (the equivalent of the old-style font size option), but that's a very esoteric way of going about it and I'd wager nobody1 does it. (In Chrome, it's buried under the advanced settings, Web content, Font Sizes. In IE9, it's even more hidden. You have to press Alt, and go to View, Text Size.) It's much easier to just select the Zoom option in the browser's main menu (or use Ctrl++/-/mouse wheel).
1 - within statistical error, naturally
If we assume most users scale pages using the zoom option, I find relative units mostly irrelevant. It's much easier to develop your page when everything is specified in the same unit (images are all dealt with in pixels), and you don't have to worry about compounding. ("I was told there would be no math" – there's dealing with having to calculate what 1.5em actually works out to.)
One other potential problem of using only relative units for font sizes is that user-resized fonts may break assumptions your layout makes. For example, this might lead to text getting clipped or running too long. If you use absolute units, you don't have to worry about unexpected font sizes from breaking your layout.
So my answer is use pixel units. I use px
for everything. Of course, your situation may vary, and if you must support IE6 (may the gods of the RFCs have mercy on you), you'll have to use em
s anyway.
Why em instead of px?
The reason I asked this question was that I forgot how to use em's as it was a while I was hacking happily in CSS. People didn't notice that I kept the question general as I wasn't talking about sizing fonts per se. I was more interested in how to define styles on any given block element on the page.
As Henrik Paul and others pointed out em is proportional to the font-size used in the element. It's a common practice to define sizes on block elements in px, however, sizing up fonts in browsers usually breaks this design. Resizing fonts is commonly done with the shortcut keys Ctrl++ or Ctrl+-. So a good practice is to use em's instead.
Using px to define the width
Here is an illustrating example. Say we have a div-tag that we want to turn into a stylish date box, we may have HTML-code that looks like this:
<div class="date-box">
<p class="month">July</p>
<p class="day">4</p>
</div>
A simple implementation would defining the width of the date-box
class in px:
* { margin: 0; padding: 0; }
p.month { font-size: 10pt; }
p.day { font-size: 24pt; font-weight: bold; }
div.date-box {
background-color: #DD2222;
font-family: Arial, sans-serif;
color: white;
width: 50px;
}
The problem
However, if we want to size the text up in our browser the design will break. The text will also bleed outside the box which is almost the same what happens with SO's design as flodin points out. This is because the box will remain the same size in width as it is locked to 50px
.
Using em instead
A smarter way is to define the width in ems instead:
div.date-box {
background-color: #DD2222;
font-family: Arial, sans-serif;
color: white;
width: 2.5em;
}
* { margin: 0; padding: 0; font-size: 10pt; }
// Initial width of date-box = 10 pt x 2.5 em = 25 pt
// Will also work if you used px instead of pt
That way you have a fluid design on the date-box, i.e. the box will size up together with the text in proportion to the font-size defined for the date-box. In this example, the font-size is defined in *
as 10pt and will size up 2.5 times to that font size. So when you're sizing the fonts in the browser, the box will have 2.5 times the size of that font-size.
Should I use pt or px?
px
≠ Pixels
All of these answers seem to be incorrect. Contrary to intuition, in CSS the px
is not pixels. At least, not in the simple physical sense.
Read this article from the W3C, EM, PX, PT, CM, IN…, about how px
is a "magical" unit invented for CSS. The meaning of px
varies by hardware and resolution. (That article is fresh, last updated 2014-10.)
My own way of thinking about it: 1 px is the size of a thin line intended by a designer to be barely visible.
To quote that article:
The px unit is the magic unit of CSS. It is not related to the current font and also not related to the absolute units. The px unit is defined to be small but visible, and such that a horizontal 1px wide line can be displayed with sharp edges (no anti-aliasing). What is sharp, small and visible depends on the device and the way it is used: do you hold it close to your eyes, like a mobile phone, at arms length, like a computer monitor, or somewhere in between, like a book? The px is thus not defined as a constant length, but as something that depends on the type of device and its typical use.
To get an idea of the appearance of a px, imagine a CRT computer monitor from the 1990s: the smallest dot it can display measures about 1/100th of an inch (0.25mm) or a little more. The px unit got its name from those screen pixels.
Nowadays there are devices that could in principle display smaller sharp dots (although you might need a magnifier to see them). But documents from the last century that used px in CSS still look the same, no matter what the device. Printers, especially, can display sharp lines with much smaller details than 1px, but even on printers, a 1px line looks very much the same as it would look on a computer monitor. Devices change, but the px always has the same visual appearance.
That article gives some guidance about using pt
vs px
vs em
, to answer this Question.
Is it okay to use negative pixels in css margin value or should I do something else?
That is fine. For example, negative margins are commonly used to center align elements.
Can a CSS pixel be a fraction?
Yes, you can specify fractional pixels. As this has been part of CSS since the very first version, it should be well supported by any browser that supports CSS at all.
Reference: CSS 2.1: 4.3.2 Lengths
"The format of a length value (denoted by <length> in this
specification) is a <number> (with or without a decimal point)
immediately followed by a unit identifier (e.g., px, em, etc.)."
When the elements are displayed on the screen, most browsers will naturally round the position to the nearest pixel when using 100% zoom level. On higher zoom levels you will notice that fractional pixel values are recognized.
Should I define CSS margins in pixels or ems? Why? When?
em
units are used for better scalability of the page when the size of the elements depend on the page's scale. It's especially important for old browsers (e.g. IE6) and mobile platforms.
px
units are used for absolute values, while em
is relative to the font size of the particular element.1em
means one font-line, e.g. you have a box with font-size 12px
that means that 1em will be equal to 12px
Also, using px
seems easier because you know the exact value, but em
units inherit the value of their container.
<p>Text</p>
<div class="box">
<p>Lorem</p>
</div>
p {
font-size: 1.2em;
}
.box {
font-size: 1.2em;
}
In this case, the first <p>
will have font-size equal to the basic font-size * 1.2, and the second <p>
will display with font-size * 1.2 * 1.2.
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