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When setting values for properties, you'll often need to set what's known as a length. Here I'm back again in the Full property table of the CSS 2.1 Property Index. I notice that as I look through some of these properties, I'll notice that often the acceptable value for them is right here in this little bracket notation known as length. So what's a length? Well, length really is nothing more than a value like pixels or inches where you have a specific value followed by the unit of measurement. Now, in your CSS, you'll write the values like this, with a number followed by an abbreviation for the unit of measurement.
Now, note that there is no whitespace between the value and measurement type. Now occasionally you can get away with leaving the measurement type off. You'll notice in the second one from the bottom, for example, 0, so if it's 0, you don't really need a unit associated with it. There is one specific property, the last one that you see here, line-height, that allows you to use a multiple. It's the only one that allows you to do this, which uses only the multiple value itself with no unit association. Now obviously, if you're not familiar with the units of measurement in CSS and when it's appropriate to use one over the other, declaring values for properties can get rather confusing.
It's easier if you think about units of measurement in terms of the way that they can be grouped. They can be grouped in one of two values: absolute or relative values--and those groupings refer to how those values are calculated. So let's take a closer look at them. Now, absolute values, which are sometimes referred to as fixed values, specify an exact measurement, and they're typically used when the physical properties of the user agent is already known. If we take a look at the absolute values, what that means becomes a little clearer. So we have inches, centimeters, millimeters, points, and picas.
So for an output device like a printer where the physical size of the output is known, these units work just fine. However, for user agents that may have different sizes or resolutions, rendering intents, or things that might change, these units aren't exactly optimal. In fact, the CSS specification instructs user agents that don't explicitly support the unit to approximate the value. So in a lot of ways, if you use these, you're going to be getting your device's best guess. So these types of units work great for printing style sheets, but sometimes not so great for screen devices.
Now, that leaves us with relative units. Now, these are called relative units because their value is relative to the length of another property. So they're really, really well suited for devices like screens where the device size or the resolution are going to be either be different or could change even in the course of viewing the site. So these units are ems, exes, pixels, grids, root ems, viewport width, viewport height, viewport minimum, and character. Now, some of these values, I'm guessing you're probably looking at them for the first time, so I feel the need to point out that a couple of these units are brand new in CSS3, and they're not fully supported yet.
Others are extremely specific, like grids. Grids are new to CSS3 and they're widely used in East Asian typography. They're referred for the layout grid used in the CSS3 text module. Other items, like say viewport width, height, and minimum, they're really cool. They're new in CSS3, and they allow you to size elements relative to the viewport. That would be the size of the actual browser window or screen. Unfortunately, they don't have widespread support yet either, so we won't really be using them. So that leaves us really with ems, exes, root ems, and pixels.
Now, I want to talk about pixels first because a lot of people think that pixels should be an absolute value, like inches or points. But the truth is the size of a pixel is relative to the display of the device that it's shown on. Now, think about it like this. If you go to your monitor and you change the screen resolution, does the monitor itself physically change sizes? Of course not! So what happens is is that the pixel changes size. So as you can see here, pixels on a 1024x768 display is going to be a lot larger than a pixel that's shown on a 1400x900. Keep that in mind when you're using pixels.
They're a workhorse. You're going to be using them over and over again yourself, but keep in mind that 16 pixels is always going to be 16 pixels, but it won't always be exactly the same size. Ems, exes, and root ems are all fairly similar. If you're a graphic designer, I'm betting ems look familiar to you. The name does indeed come from typography, and it roughly means the value of text at its default size. However, the value of an em changes depending upon where it's used. If it's used to define the size of a font, then 1 em is equal to the value of its parent element's font size.
Here's a good way to think about this. If within the browser, somebody chooses 16 pixels to be the default size of the text, if you're not sizing it anywhere else, that means that 1 em would equal, say, 16 pixels. What this means is that you could set one value for the body element and then set all headings and paragraphs relative to that value. The added beauty of this technique is that the starting size of the text is up to the device itself. So whether it's a desktop browser or a mobile device, or even if the user has changed the font size to compensate for, say, a vision disability, the overall font size will get bigger or smaller, but the relationship between those elements is going to stay exactly the same.
Now, if you're still with me, you probably remember me saying that the value of an em changes based on where it's used. Let me go into that in a little bit more detail. So if an em value is set outside of font-size, it's equal to the computed size of the text of that element. I know that seems confusing, so let's take a look at how this works. So let's say our device font size is set to 16 pixels. Our heading over here has a font size of 2 ems, so that would be 32 pixels. That would be the computed font size. Now, the margin-bottom is set to 1 em, but because the computed font size is 32 pixels, the bottom margin will also be 32 pixels.
So it really matters where the em is used, whether it's used to define the size of the font or outside of that. So you have to remember that, that's something that you really need to keep in mind, because this can trip you up if you're not familiar with how this works. So because of that, ems might take a little while before you become really comfortable at using them, but once you are, you'll find that they're incredibly flexible unit of measurement that you're going to use a lot. Now, as for the two remaining relative values, exes and root ems, they're very similar to ems. Exes are a lot like ems. They're just based on the x-height of the font.
And root ems, I'm really intrigued by these. They're an addition to CSS3 as well. One of the really big problems with using ems is that they're always relative to their parent, so the more nested your structure goes down, the more child elements you have, the harder it is to try to figure out exactly what size something is supposed to be. Now, root ems are, on the other hand, they always size text relative to the root element, or in most cases the body tag, so they give you a much more consistent starting point for sizing text and other elements. Now I need to point out that support for root ems isn't complete yet, but hopefully we'll be able to start using them in the near future.
I want to mention one last unit of measurement that's not really considered a length. So even though I always consider it a relative unit, it's not really categorized as a relative or absolute value, and that would be percentages. Now, percentages are exactly what you think they are. If you size an element width to 80%, for example, the computed size will be 80% of its parent element. Text size with percentages calculates the value based on its parent element as well. Percentages are great for building things like fluid layouts that react to the size of the screen or the device on which it's viewed.
Keeping track of all the various units of measurement in CSS can be a little tricky, I will admit. But just like most things in CSS, the more you use them, the more familiar they're going to become to you. Unless you're dealing with really specific circumstances, you're most likely really going to use a mixture of pixels, ems, and percentages for screen designs, and inches, points, and picas for print design. Now, either way, you should be aware of what your options are when defining lengths for CSS properties.
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