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what is the difference px,em and ex?
I have a question for all the coding geniuses on StackOverflow.
I am a newbie, and I am about to start building my third website. Being that I had some problems with the layouts of my first two websites, I am asking this question before I start on my third:
What measurement is it best to use for
the css elements? Percents,EM's or Px?
Which form of measurement will ensure that I have a site that will not get distorted on different browser sizes/resolutions? Is there anything else that I have to be careful of when building my site so that it will not get distorted when a user zooms, or looks at the site from a different browser size/ resolution? (as was he case on my other sites)
Thanks for your time, guys. Any help would be greatly appreciated!!
Thank you.
Whichever is easier for you to work with.
Modern browsers (i.e. everything in use today except for IE6 and IE7) have a concept of "CSS pixels" which is different from "actual pixels," so e.g. zooming changes the size of a "CSS pixel." Fonts will scale just fine; if you say the font is 14px, it will start out that way, but if the user zooms it'll get bigger. Thus, if it's easy for you to measure in pixels, for example to size page elements relative to an image of a given pixel size, you should do pixels.
Sometimes you want to size things relative to text, though. If the width of an em-dash is a useful measurement, somewhat representing the "longest possible character," go ahead and use ems.
And finally, if you're trying for a fluid layout, percentages can be great: a gradient that starts fading 50% across the page is often what you want, as opposed to one that starts fading after some fit number of pixels. Even if you're not fluid, and the width of your container is fixed to e.g. 900px, it's still often useful to say "this goes at the 50% mark" or "I have one thing at the 33% mark and one at the 66% mark." That's much easier to work with than figuring out what the corresponding pixel offset is every time, and makes your intent clearer to anyone reading your code.
Short answer: it depends.
Longer answer:
There is a place for all three units, frequently in the same design. There is no "best" unit; they serve different purposes.
Pixel units generally offer the most precise control over the size of the elements in the user interface, but also restrict that size such that it does not change with regard to the other elements of the page design. The size of pixels themselves may change. For example, a Retina display packs more pixels into the same physical space as a non-Retina display, so images which were designed for traditional displays get scaled up. Similarly, traditional desktop web browsers may adjust the size of pixel in response to the user zooming the size of a page. In these cases however, the pixels change sizes throughout the entire document, and retain the same proportions with regard to one another, so you can use px values and expect them to work sanely in most conditions.
EM units vary according to the size of the text. They're most commonly used for setting the size of text, and for line heights; but there have been some interesting things done with "elastic" layouts such as the elastic lawn zen garden (turn off page zoom for this site; switch to text-only zoom and change the size a few times).
Percentages vary according to the size of the containing element, expanding and contracting depending on how much room is available to them.
And, really, it's very common to see web designs that use all of these. For example, suppose you have a site with two columns. The main column must expand and contract with the browser width, but the secondary column needs to stay the same width. The main column might have a width of 100%, but also a margin set in pixels for the secondary column to float in. And the text and line height might be set in ems.
So, the real answer is: they all have their uses. Keep practicing, and pretty soon you'll figure out how it all fits together.
EDIT: In the example above, I should have said "a width of auto" -- meaning take up all available space after margins, padding, and borders are accounted for. Sorry, I tend to think of that as a percentage even though it's actually a keyword.
from accessability point of view need to use EM's. You need you layout to adapt to very different fonts sizes so if allmeasurements are in EM's everything will scale as accessability tools increase font size
When creating liquid layouts, it makes sense to use percentages for the widths of your blocks, so they shrink and grow with browser size changes.
For heights, pts have a specific spatial value, and em are related to your current point size. This is useful because things specified in those units will be roughly the same size on everyone's display (unless they have different zoom factors applied). em are also useful when working with a dimension driven by an amount of text.
Browsers will also scale values specified in pixels, so they are not any more a "trap", but they are rarely the "natural" choice for layouts, unless working with raster images.
As Will and Domenic say: use all three, when appropriate. As you get more experience, you'll get a better feel for when to use which.
If you want to design a fixed layout website then use px or em.
If you want to design a fluid layout website then use percentages.
Percentage is always relative so page content with dimensions in percentage will automatically resize on window resize and on different screen resolutions.
px and em are one and the same thing. Same in their nature. They define the absoluteness of the dimensions. Btw for the difference, 1em=current font size. So if your html or body has css font-size:19px; then 1em=19px.
Related
I'm redesigning my site because looks awful on different resolutions (apart devices), most guides and tutorials rely on % and em than fixed values. I'm still learning this so I'm reading everything around.
Thought this would solve the question with different display sizes but again: we have to craft some more CSS for fix some specific issues.
If I need to add media-queries for extra display sizes, why use % then? Do use % really reduce coding? Is ok need to add some extra css for some sizes or am I doing something wrong?
Thanks for any advice!
The purpose of using em sizes is to allow the base your design off of the user's choice of font size. I may use a larger font size because I have a huge monitor and poor eye sight, while someone else might prefer a smaller font. By using em units, your design will accommodate both of our font preferences and resize accordingly instead of forcing the font size to a given standard (eg. "12 point font").
In a similar manner, percent (%) units allow your design to respond to different browser sizes. Used in conjunction with em units, this will allow text-based elements to respond to arbitrary font size choices, and layout elements to respond to arbitrary browser sizes.
It is perfectly acceptable to design a single responsive design for all media types. Media queries are intended when you want different display styles on different devices, not to "support" different display sizes. An example would be to use serif fonts on print media and sans-serif fonts on display media, since usabilities studies have shown that these font faces are preferred for these sorts of media.
Furthermore, it allows you to do custom styling for some situations like mobile devices, where you may want to consider that the user has a limited amount of bandwidth and maybe cut down on extra images. Or if you want to display your content in a completely different layout for the microscopic screen afforded by certain phones.
% allows your site to be responsive to the user's method of viewing. Let's say you have a div that's at 100%. It'll fill the entire section, regardless if it's on a phone or desktop.
It should be okay to add extra CSS for sizes as well. As far as I know, you can have some elements display in % and some with a fixed px value, although they might conflict depending on how the page is setup or what it is being viewed with.
For example.
Your website header have the width of 950px; But in a mobile device, it may fit at 450px;
So, you use media-query to reposition some elements and handle some size issues and set the header width to 450px;
But, if you use % values, you can set your header div to have 100% of width base in its parent. So you can only change the body or some container div width, the all childs going to adapt.
Diana,
I am glad I came across this question. I literally just uploaded my first responsive design which is 90% based off of percentages when it comes to font-size and widths.
Check out the below:
http://www.noxinnovations.com/portfolio/responsive/
Obviously, it doesn't look amazing, and the image looks way out of place... But do me a favor and resize your browser window, by slowly making it smaller and smaller. I did that by setting a percentage width!
Trust me, I tried doing this responsive design test with pixels, and it didn't turn out too well. The percentage width ensures that regardless of the resolution and or pixel dimensions (per se) the design will always cater to the size of the screen. Also, I did not have to use one CSS3 Media Query, but I would highly suggest using CSS3 Media Queries only when you need them.
In my opinion, I should probably have a Media Query for a larger screen.
I hope this helps you as much as it has helped me!
Thank you,
Aaron
When creating a web-site design, is the only real option to provide a fixed content width?
I notice most major websites (this one included) center all the content into a fixed width, which ensures all elements look correct on all screen sizes.
I think I already know the answer to this, but a colleague seems to think that there should never be a horizontal scrollbar and a website should resize to the users screen width. I make the argument that text can resize but other elements can’t (buttons, textboxes etc), so a fixed width is the only option.
I'd love your opinions each way.
Surely fixed width is the only way you can guarantee the correct layout on ALL browsers?
You can get clever with floats/media queries to make layouts work well at narrower widths.
But it is indeed more common to go with a fixed width:
It’s easier to design
Not many people have narrow monitors these days — most websites seem to assume they’ve got 1000 pixels of horizontal space to work with, and I haven’t heard of them getting many complaints
Touch-based OSs (iOS, Android et. al) make zooming web pages feel very natural, so pixel dimensions aren't quite as important there.
When lines of content get too wide (I think more than around 60 words?), they get difficult to read, so there’s not much advantage to be gained from having elements expand to full width for users who have bigger browser windows.
I think the most likely group of users to have less horizontal space available is smartphone users, and you’re going to want a specific design for them if you care that much about their experience.
Oh, just one thing though: “Surely fixed width is the only way you can guarantee the correct layout on ALL browsers?” With the greatest possible respect, that’s a bit of a programmer’s way to think about it. You can’t guarantee anything about web page rendering. Browsers and operating systems can do whatever they want. That’s the web. Let it go.
Dynamic-width web-sites are generally composed of one or many percentage-width blocks which make 100% total. So blocks size change with resolution or browser resizing, but if total width is too short it can occure lisibility problems, so a minimal width is specified for <body>, in which case a horizontal scrollbar appears.
But you can think differently : http://www.smashingmagazine.com/2008/08/14/will-horizontal-layouts-return/.
I tend to make my websites have a minimum width for legibility, but otherwise the width is determined as a percentage of the window size (I normally pick 80%).
I find this gives me ample coverage of the viewport, whilst still looking good on larger monitors.
I find this approach fails on larger screens when there is little content, as the content tends to get stretched into a line at the top of the page, which can make things confusing. So for those pages it can be good to have a fixed width, centred style.
No. A thousand times no.
If your website presents any kind of body text, fixed width is an abomination from the depths of the mid-90s, forcing readers to scroll like crazy, while being mocked by vast expanses of unused screen space to either size. (Now, a variable number of columns I might buy.)
BTW: flag for CW.
Horizontal scrollbars are a big problem. A properly designed website should be able to scale down to be used on small resolution screens (640x480). Also, it's nice to have to have my browser window maximized just to browse a site. Also, there's no reason why a person with a high resolution monitor shouldn't be able to use the extra space their monitor gives them.
I read somewhere on a site :
In principle, using a px measurement for font-size is not a good idea. A handful of browsers will prevent the font from being resized by the user if you do this.
Is this right??
Is this right??
Well, yes, certainly there are a few (generally older) browsers that won't let you resize the text when it's sized in px, pt, in, mm and so on.
But even in browsers with a working zoom, it's polite to work relative to the user's stated preference for font size, so they don't have to resize the text or zoom the page manually to make it comfortable. (Fixed fonts plus fixed width page can be particularly bad for this in page-zooming browsers as zooming up is likely to make the columns of text too wide to fit the screen.)
In principle, using a px measurement for font-size is not a good idea.
I wouldn't go so far as to say it's always a bad idea.
There are often elements on the page where you want the text to be sized to match an image (say, a header with text below it that should fit more-or-less without wrapping, or text above a background image made to fit it). In that case you should use px to make fonts and images line up nicely.
For the page's main body text, yes, it's nicer to use em/% and let the user decide the size. But for text that plays a part in the page's graphical layout, px fonts are typically the best bet.
The issue is mainly with aspect ratio. Let's say my resolution is 1024x768 for an aspect ratio of 1.33. For 1920x1080, the aspect ratio becomes 1.77, meaning that any image displayed at a specific pixel width and height will be a different size in inches due to the scaling effect of the aspect ratio. The same basic issue exists if you zoom as you are effectively using the aspect ratio in the zoom window.
Most websites get around this by using em which, to be honest, suffers from some of the same downfalls as any other rendering mode. Even "device independent pixels" are based on the DPI of a monitor. So, use em knowing that it just has less faults than px, not because it's perfect.
Read the following article if you want a more in depth study.
http://www.w3.org/WAI/GL/css2em.htm
Yes, at least IE6 & IE7 do that. Take a look at How to Size Text in CSS.
Quoting CSS: The Definitive Guide by Eric Meyer
There is one more value that is
potentially the same as 36pt, and
that's 36px, which would be the same
physical distance if the display
medium is 72 pixels-per-inch (ppi).
However, there are very few monitors
with that setting anymore. Most are
much higher, in the range of 96ppi to
120ppi. Many very old Macintosh web
browsers treat points and pixels as
though they are equivalent, so the
values 14pt and 14px may look the same
on them. This is not, however, the
case for Windows and other platforms,
including Mac OS X, which is one of
the primary reasons why points can be
a very difficult measurement to use in
document design.
Because of these variations between
operating systems, many authors choose
to use pixel values for font sizes.
This approach is especially attractive
when mixing text and images on a web
page, since text can (in theory) be
set to the same height as graphic
elements on the page by declaring
font-size: 11px; or something similar,
as illustrated by Figure 5-15.
Using pixel measurements for font-size
is certainly one way to get
"consistent" results with font-size
(and, indeed, with any length at all),
but there is a major drawback.
Internet Explorer for Windows up
through Version 6.0 does not allow
users to easily resize text that has
been set with pixels. Other browsers,
including Mozilla, Netscape 6+,
IE5+/Mac, Opera, and even IE7, allow
the user to resize text no matter how
it's been set. Thus, using pixels to
size text is no more of a guarantee
that it will stay the same size than
is any other method. The other
approaches discussed in this chapter,
such as keywords and percentages, are
a much more robust (and user-friendly)
way to go, as they can be used to
scale text from the user's default
font size.
That said, the issue here is what unit to use for your body element. In most cases, you should use the em unit for other elements like headings and paragraphs.
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 11 years ago.
Is it bad in terms of compatibility to use pixel numbers in CSS instead of percentages? How about lower resolutions? Is it okay to work with them in ranges of 1-100?
All of the measurements have their own purposes:
Use pixels for pixel-based things, like borders. You probably don't want a border that ends up being 1.3422 pixels wide.
Use text-centric measures (em, ex) for text-based things, like content areas, labels, and input boxes. It's an easy way to make sure you have room for text of a certain length and width.
Use percents for window-based things, like columns.
There are exceptions, of course. For example, you might want to specify a minimum column width in pixels. But follow the above and your pages will scale well. ALWAYS zoom in and out on your pages to see how they work with different font sizes and browser shapes -- don't get surprised later.
This is a difficult question, because the answer mostly depends on your situation.
Pixels are not that bad, I mostly use them too. (Sometimes even for font sizes.)
I usually fix the outer block element of the layout by a given size (pixels with fixed-width layouts, and percentages with fluid layouts), and on the inside elements I usually set percentages whenever possible.
There are some elements which simply can't be styled with percentages or ems, especially the more fancy stuff coming from graphical designers who don't understand such principles.
For example: if you have a column on your site with a simple style, you can set its width to a percentage easily, but if it has a background image with a specific width that is not designed with scaling in mind, it only looks good with a fixed-width. In such cases, you'll have to ensure that the rest of the page occupies the remaining width correctly.
Note that you can use pixels with percentages together.
For example, this is a snippet from one of my latest web apps:
min-width: 800px;
width: 80%;
max-width: 1500px;
The choice also depends on what design or layout you would like to achieve.
For a fixed-width layout, pixel values are fine. If a designer gives you a Photoshop image that contains really fancy stuff which would be extremely complicated to even think about how it would resize, you should definitely go for this.
If your layout needs to be dynamic, you should use percentages to make sure that it expands as the resolution changes, and you can use the above code snippet to make it look better in scenarios where it would otherwise look insane.
Some layouts (eg. imagine if StackOverflow would take up all the space) would look pretty ugly on a width of eg. 1920 pixels - the line widths would be so insanely high that it would be extremely unconvenient to read.
This is what max-width is for. Even in some dynamic layouts, you'll have to limit the maximal width of your site to maximize usability and readability.
And also take into consideration the smaller screens.
It is true that noone uses a 800×600 desktop anymore, but many people browse the web with mobile devices which have even smaller resolution.
This is what min-width is for: to make your dynamically expanded layout look less crowded on smaller resoultions.
I hope this helps.
EDIT:
The Smashing Book has some very nice thoughts about the subject.
EDIT 2:
I don't want my post to sound like I want you to force pixel-based sizing on your visitors.
(Apparently, some people in the comments misunderstood me in such a way.)
To clear it up:
I believe that the ideal layout is one that adjusts well to any possible resolution or setting.
However, we can't always do everything perfectly. Time/resources and the target audience are the key to determine if your site requires that advanced functionality or not.
I'm suggesting that you use the right thing for the given job.
If you are developing a site which will have a significant percentage of visitors who require more advanced adjustments to the site, it may be well worth it.
(Of course, sometimes we just do it for ourselves to have the feeling of doing things the right way, but is is not always a financially sound decision.)
Still, you should do the proper research about what sort of site will be it, who will be the visitors, and such stuff, before deciding about layouts, and whether it is worth the time to make them fluid or more dynamic.
Font sizes
I think you must first understand the issues that exist with working with pixels in CSS:
Zoom in older browsers is broken. For example, IE6 and IE7 do not resize text when zooming. Line-height can be quirky too. These problems do not exist in modern browsers, but they are a reason why many shy away from using pixels for font sizes.
Everybody will see text the same size if you specify the font size in pixels. Browsers have a default size of 16px for paragraphs, so if you only use em and other relative sizes, you will respect the decision of users who change this. This is especially important on text heavy sites, especially if there are more older users. On the other hand, if the design of a site is important I think it is possible and justifiable to use px to specify font sizes without breaking usability.
In the end, you need to make the decision yourself, and it does depend on the exact circumstances, but I think that specifying font sizes in pixels is okay.
By the way, when working with em to specify font size it is a good idea to set the body to font-size: 62.5%. This means the base font size is 10px, so 1em is 10px, 1.6em is 16px and so on, making it easier to think in pixels while designing using ems. I still find it frustrating to work like this, especially when the values of ems cascade. There are some very handy sites like PXtoEM.com that help with this.
Layout issues
The screen is a pixel based layout, so pixels are an intuitive choice for many things. The main issue here is that different users have different screen sizes. As others have pointed out, using min-width and max-width in pixels along with width in percent is a helpful way to respect the size of the screen, while preventing your site to be unreasonably squished or stretched on very small and very large windows.
However, I would generally avoid this approach in favour of CSS media queries. You can then use fixed width chunks and make the layout wider (amongst other things) as the screen size increases. However, CSS media queries, like all cool web technologies, suffers from lack of browser support. Most notably, IE8 and earlier do not support them, although there are JavaScript fixes. On the other hand, the iPhone and other handheld devices do support them, and I would strongly recommend them if you want your site to look nice on these devices.
I think fixed width grids are fine. Fixed width grid systems like 960 Grid System
are popular enough in their own right, and there are so many other sites that have a fixed width, that I doubt you would hear many complaints if you did this. Handheld devices that do not have large screens are an issue, but this is where CSS media queries should be used, so it is possible to specify everything in pixels and have your site looking beautiful on the desktop and on the iPhone.
Conclusion
Ultimately, everything depends on who your users are, what you need to support, and what you want your site to look like, but there is nothing inherently wrong with using pixels in CSS.
That depends on what you are styling. For columns for example, the width should probably depend on the text size to ensure that it will look optimal on multiple resolutions/screens. If you want to divide your page in two parts, you should use percentages. But if you want a 1px border between these two parts, independent of the resolution, use pixels.
Basically it depends on who's hiring you and consequently the audience of your work.
For institutional purposes (where content should prevail over the shape, like a goverment project), you better work with .em or %, they are harder to control, but they will be really user friendly in terms of accessibility.
If we talk about corporative Websites (Where shape is the deal) pixel will be a more accurate tool to fulfill your customer expectation regarding his brand.
A liquid interface (%, .em) is always a good stuff when it is smartly done, but don't forget to check your design under extreme conditions and be sure that it will be stable.
If you work with pixels, you will have absolute control over the final look of your site, but you will have to assume the impossibility for some user's to operate with it efficiently.
Best option: rather than designing a Website compatible for all platforms (what will result in a multi-deficiency design) suggest your customers special versions of the site adapted for every demand, what is a better practice and a better business for a designer too...
I would say avoid it if possible except for in certain cases.
For instance for a thin border it's ok to just specify 1px.
Also for max- and min- style attributes it is ok. But then make the non-max/min attribute be a percentage if possible.
This really is a good question which I have asked many times before. Not being a hardcore web designer (I'm more on the development side) I've usually asked the designers I work with about their opinions, and here is what I have distilled:
Using percentages vs pixels for sizing elements and so on is really a matter of personal taste or the requirements in each specific case. If you need it to scale, or it will scale well, use percentages. If not, use pixels. People here have used the examples of major columns in a page maybe needing to be fluid but borders may well need the precision of a pixel measurement.
Obviously sizing images is pretty much tied to their resolution and pixel units, so I always use them in this case.
However, using em sizing versus pixel or even point sizing for text is a whole different kettle of fish. Most of the guys I work with have a base reset style for font sizes which brings the size of 1em down to about 12px. They then use ems everywhere else (or as near to everywhere else as is possible) to size text elements, form controls and the like. This is how I operate now too as it seems to work well across a variety of browsers, OSes and DPIs on desktops and laptops. I can't vouch for mobile devices though.
Accessibility is the key though - if you need to make something usable for people with disabilities or work on a variety of devices out the box, even things you may consider old, then scaling will be a requirement. Build it into your model for designing the site from the get-go and you may realise that absolute pixel sizing is not even needed in that case.
For example, an art-heavy design is probably going to be designed at a single scale, but the new educational website for disabled users is going to have to work in a variety of situations.
Just remember, the W3C put a variety of methods for sizing and scaling into their specifications for a reason - flexibility. Do whatever fits best and works well for your audience (as Moustard said earlier).
I'm not a css/html expert but convention I use is to use pixels for the outer containers and percentages for the inner objects.
Another rule of thumb I have with anyone reviewing my layout is a Three Pixel Rule. It is rare that everything will line up exactly within every browser out there. We've agreed the effort outweighs the benefits to move anything three pixels or less.
Do the right thing by default. If a user visits your site with javascript disabled and no preference cookies, you should serve them a web page that is as functional and accessible as possible.
Your default stylesheet should assume nothing about the browser window, and still render a functional page, however hideous, down to 320x320 resolution, and up to unbounded sizes. If photographic fidelity is so important that you can't trust browser scaling, and sometimes it is, then you should use pixel measurements (at least minimums) for elements encapsulating images of fixed size, to make sure that you don't end up with important things being covered up. Specifying minimum widths along with percentages should give you the control you need while giving your users the accessibility they need. Even if accessibility isn't an issue now, it will be later if the site is successful.
When you do know more about the browser window, then it's okay to use fixed widths in alternate stylesheets, if the importance of preserving the design justifies the added work.
If you're targeting a fixed size then pixels are fine. But if you want your layout to look good on various resolutions/screen sizes, then you should stick to more relative measures like percentages. Most folks are targeting the latter.
It is ok if that is what it takes to please your client. Most of the time, you can't balance the amount of space in diverse screen sizes with only percentages. CSS3 media queries helps here but adoption is still problematic.
I prefer pixel in most cases.
For example - default width of content ares: 960px.
In "1024" it will be ok, with small free spaces on the both side of the screen. Put some gradient or background there. In "1280" or "1440" or "1680", or whatever - there will be bigger and bigger gap, but it will look nice.
Yep, for 800 - this site will be ugly. But who cares? Every major players dropped support for 800. There are too few people with this resolution..
There are many various sites, where %% will be much better solution, though.
The building blocks of a screen are pixels. You can't go wrong with fixed pixel sizes. As other posters have mentioned, you can also use percentages or "em"s for a scalable substitute.
My designer keeps throwing out the term "liquid" layout. What does this mean?
Thanks for the clarification, I have always just called this a percentage layout, and thought he was saying that the pieces could be moved around, and that was liquid
A "liquid" layout is a site layout that expands to fill the entire available area as the browser window is resized. Typically this is done using CSS. Liquid layouts can be quite helpful for certain types of sites, but they also tend to be significantly more effort than fixed width layouts, and their usefulness depends on the site content and how well implemented they are.
From http://www.maxdesign.com.au/presentation/liquid/ :
All containers on the page have their
widths defined in percents - meaning
that they are completely based on the
viewport rather than the initial
containing block. A liquid layout will
move in and out when you resize your
browser window.
Basically, it's a layout of a web page that doesn't rely on a specific width specifications for elements in the page.
See the discussion over at Wikipedia.
It means a layout which adjusts dynamically to the browser (or whatever client) width and height, to make efficient use of all available screen space, as opposed to (mostly) fixed width layouts which are made to fit a common denominator resolution at that particular time (e.g. 800x600 used to be the norm for websites for many years).
See this:
http://www.time-tripper.com/uipatterns/Liquid_Layout
Liquid Layouts refer to the design concept of a website. A liquid layout will move in and out when you resize your browser window, due to is having percentages and relative widths in the CSS.
It just means that it will contract/expand to fill the browser's window size (usually the width), up to a certain point if things are done well. Otherwise text can get quite hard to read on big (24"+) monitors.
One of two:
The design will scale to the width of the browser (as in, if the browser was 1024px wide, the design will be as well)... although this does get quite fun when designing for 100px wide browsers (sometime designers will actually set a min-width though).
The design has a fixed width, but is set in a measurement using a relative size... for example "em"... so as the font size is increased, the width of the page increases.
A liquid layout is a method of CSS layout that defines all widths in percentages, so the areas of the page will grow/shrink when the viewport (browser window) is resized.
They're very useful if trying to create a site that will fit both large and small screens. They're a little more difficult to work it than fixed layouts, because you're relinquishing some level control over how everything fits in the page, and you have to pay very close attention to your content, to make sure it doesn't fall apart aesthetically on resize.
I would say liquid layouts are most useful for text heavy sites with a fairly basic column layout. You might also find a happy medium with an 'elastic' layout -- one that has both liquid and fixed areas.
In a true Liquid layout, your content expands and contracts to fit your user's browser window in a meaningful, calculated and intelligent way. So it's more than just setting your column and container widths to percentages.
Done well, this can result in a increase of perceived quality. Done poorly, it's a usability nightmare.
Going Liquid is a huge pain the rump. The pain is worth it though if the topic/client/product(s) you are building the site for have a strong visual quality to them (think summer blockbuster film site), require a certain fit and finish, or if it needs to display large chunks of data.
Note: I'll update this a bit later with links to good examples and citations for my claims