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A little earlier I talked about multi-serving content where you deliver a web page to both a mobile device and a desktop device where the content is just styled appropriately for each one of the form factors. And the way that you accomplish that is by using a technology called CSS media queries which is what we are going to look at now. CSS media queries provide a way of selectively applying style sheets based upon the characteristics of the display media where the content is going to be consumed, whether it's a screen or a printed page or so on.
But what's nice about it is that you can do things like look at the window size or the screen size or whether or not a device is in portrait or landscape orientation, for example, and apply style sheets based upon those kinds of criteria. CSS style sheets are a W3C candidate recommendation as of July 2010, and you can see I have provided the link there to go and learn more about them. But for now let's just go ahead and take a look at the most common stuff that you can do with media queries.
The most common things you want to do with media queries are displayed in this table here, and right at the top you can see there is properties like width and height, and those properties indicate the width and height of an output surface, like a window, of width or height units, pixels, centimeters or whatever. Below that we've got device-width and device-height, and then we've got orientation. These are the most commonly used media query features that you will run across. And for features that use units you can specify any CSS-compliant unit, like pixels, centimeters, inches, ems, so on.
Now the ones I have listed here and many other media query features except a min or max prefix, and the reason you want to do this is because if you use a media query feature like width that basically means target, say, window of exactly width units. So if I said width: 800, the window would need to be exactly that wide in order for a style sheet to apply. So what you typically do is use things like min-width or max-width, and the min prefix and the max prefix will work for everything listed here except for orientation.
Obviously, there is no such thing as min-portrait or max-landscape; you are either in portrait or a landscape. Okay, those are the commonly used ones. Here are some less commonly used ones you might run across, things like aspect ratio, which is the ratio of a display surface's width to its height. So these are typically used on televisions, 4/3 for old TVs, 16/9 for HD TVs, and so on. You can also look at things like color and color-index and what the resolution of the screen is.
And again, you can use the min and max prefixes with all of these except for the scan and grid media query features, but we are not going to focus on these because these are not very commonly used. Okay, let's see how they are used. To use a media query, it's usually used in one of the following situations. Now on the desktop what you typically have is a screen with a window in it, and the window has properties like width and height. And in media queries the width and height properties would refer to the width and height of the window in the desktop context.
And to get the device-width and the device-height you would use those media query features, and that would refer to the screen. Things are a little bit different usually on mobile devices. Mobile devices don't normally have separate windows. The display window that you're seeing is the same size as the screen. So in that case you have a screen size like this and usually height and device-height and width and device- width refer to the same kind of thing. So that's the main difference between using media queries on desktop and devices.
Now to define media queries there's a couple of ways you can do this. You can define a media query directly within a link tag, as you see here where I've got a link tag that's referring to a style sheet, and that bold text there is the media query itself. Now you have probably seen media queries used before without that 'and' clause in there. What it probably usually looks like to you is link rel="stylesheet" and then media= and it will just say, "screen", or it will just say, "print." The and part where it says "and (max-width: 800px)," that's where the query part comes in because the media query is asking the display surface, "Hey, how wide are you?" If you are at maximum width of 800 pixels and the width of the display service is below that right now, then apply at the style sheet.
You can also do it within the CSS style sheet using the @media directive. So in this case I've got an @media directive where it says screen, so I am targeting screens. And the min-width where this style sheet would apply is 801 pixels. So if the screen is greater than 801 pixels wide, or including 801 pixels, then the media query would evaluate to true and the style sheet would apply. Below that level the style definitions contained within those braces would not apply.
You can define multiple CSS media queries for multiple form factors, and I'll show an example of that right here where we have in the first part we have styles to use for small screens, such as phones. And I have a link going to the style sheet. This is media="screen" and the maximum width is 480 pixels. So for screens up to 480 pixels, we would apply the phone.css file. Then for tablets we have the same kind of definition, stylesheet, and in this case the media is screen and the minimum width is 481 pixels and the maximum width is 800 pixels, because most tablets are between 480 and 800 pixels wide--at least when they are in portrait mode. And if that query evaluated to true then we would apply the tablet.css file.
And then finally, for larger screens we would have media="screen" and just a min-width definition of 801 pixels. So for screens that were wider than 801 pixels, or I should say 801 pixels or wider, we would just include the styles.css file. So the actual question might be, should you use width or should you use device-width? Well, the width and device-width properties mean slightly different things. The width property refers to the current display surface and on a desktop that means the window.
The device-width property refers to whatever the screen display size is. So what that means is on a desktop where the window can be different than the screen, the width property can change. The device-width property doesn't typically change. The general guidelines to follow are this. If your style sheet is going to be used both on desktop and mobile devices, use the width property since on desktops the display service width can change.
If your style sheet is only going to be used on a mobile device, you can use the device-width property, but generally speaking I just use the width property because it's sufficient for most needs. Okay, let's take a look at how this works in real code. Okay, so here I am in the code, and I have got my mediaqueries_start example file open, and this is the HTML version. I have also got mediaqueries_start, the CSS file, and I've got my ExampleSnippets.txt file. So let's begin by taking a look at the mediaqueries_start file.
You can see that here's the code, and let me show you what this looks like in a browser. So here's the file in a desktop browser, and you can see that as I change the width, nothing is happening, and I am just getting the same content when it's flowing, and that's not the experience I would like to have. Let me show you what the finished example looks like. Okay, so here is the finished example. You'll notice that as I change the width of the window down to about 800 pixels, something is happening there, so you see that the links are going horizontal instead of being vertical like they are here.
The title space is changing both right alignment and background color and foreground color, and you can see down here that there is a footer, and the footer in this page looks a little bit different than it does in the other page. So let's see how we got that to happen. So here in my start.html file I've got a mediaqueries_start.css file that I am including. So what I am going to do is go over to the media query CSS file and I am going to go to my snippets and I am going to copy this code right here.
I am going to copy @media screen and (min-width: 801px). I am going to paste that in up here, and I'll put the closing brace down here. Okay, so what I have done is defined a media query where I'm looking at a screen, media, and the minimum width that these style rules have to apply is when the screen is 801 pixels or larger. So when the window is that wide or larger the rules contained within this media block will apply.
Let's go back to the snippets. Now I am going to copy the other media query, and that's this code right here, all the way down to the last brace, and I will copy that and I will paste it below this one. So now, I've essentially got two media queries: one where I'm looking at the minimum width is 801 pixels or larger and then another one where the rules apply for the screen where the maximum width is 800 pixels.
So if the window width is 800 pixels or less, these rules apply; otherwise the other rules apply. I am going to save this, and I am going to make sure this saved as well. And now I am going to go back out to the file system, and I'll start up mediaqueries_start, and we will refresh this and we will go ahead and change. And you can see that now as I am changing the window size, you can see that the layout is changing. Okay, now let's make sure that this is working in our mobile emulators. Okay, so now what I am going to do is copy the files that I just edited over to my web server.
So I will copy them in here, and now I will try it out in my emulators. So let's try it out in the mobile Opera emulator first, and we will do the localhost/exercise_files. And you can see that because the screen width is less than 800 pixels, the style sheet for the small size is being applied. Let's try the same thing in the Windows Phone emulator, localhost, and we are getting the same results over here.
So that in a nutshell is how you use media queries to change how style sheets are applied based upon the physical characteristics of the device that the content is being rendered on.
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