Join James Williamson for an in-depth discussion in this video Column height considerations, part of CSS: Page Layouts (2012).
In the previous layout examples we've been forcing our columns to be fixed height so that we can visualize our sample layouts a little bit better. Now, in practice, the height of columns is typically determined by the content inside of them. Now, most of the time designer simply factor this into their design and build layouts that work with a natural height of columns. But what happens if you need a column to stretch to match the height of another column, or if you want it to take up all the vertical space available? Well, in those cases you're going to have to carefully plan your layouts around those requirements, and you'll need to understand exactly how browsers calculate the height of elements.
So before we can get into specific layout techniques around controlling element height, I want to take a closer look at how browsers calculate height itself. If I scroll down, I can see it's a very simple structure. So inside the body tag we have a section here, and we just have a paragraph inside of that. If I preview that in my browser, you can see that right now the body's width is set to 960 pixels and the section's width is set to 50%. Now, the reason for that is, once we start stretching these columns, I want you to be able to tell what's the body and what's the section.
Now, most people's misunderstanding of height comes from them understanding what happens with width, and let me show you that really quickly here. So I go into the file and I remove the width of the body and I remove the width of section and preview that in a browser, what happens is both the body stretches to 100% and any block-level content inside of it, in this case the section, stretches to 100% as well. So the default for block-level elements inside the browsers if no width is defined, they're just going to stretch out to be 100% the width of their parent element, in this case the entire viewport.
So a lot of times designers, when they set height to 100%, they're expecting the same behavior, that the column will just go ahead and stretch to the height of the window, and when that doesn't happen, it kind of confuses them, so let's go and take a look at that. So I am going to go back into my code here, and I am just going to undo those widths. I sort of need those so that we can see what's going on. I'm going to come down here to section and I am just going to set section's height to 100%. So I am going to save that, preview that in my browser, and bam, nothing.
Well, that's not absolutely true. So what's happening here is that the height of an element is set to auto by default. What that means is is the contents of the element defines its height. So in this case, body's height is being defined by the content inside the section. So when I tell the section to be 100%, it's saying be just as tall as the body. The body is getting the height from the section, so in essence, we're saying be 100% of yourself.
It might be fulfilling, but it doesn't work the way we anticipate it. So what we have to do is we have to go in and set explicit heights on every single parent element above the one that we are targeting. So if I come in for body and I say height 100%, unfortunately, that doesn't work either, because there is another tag above body and, yes, that's the HTML tag. So if I come here and say, okay, height 100% on the HTML tag as well, now if I save this and test it, yay, everything is stretching, including the body and including our section.
Now, all seems well, but it just sort of seems well. Remember, it's stretching to the height of the viewport, which means the height of the available space, so what happens if the content is longer than that? So if I come down, for example, here to my section and I grab this paragraph and copy it. Let's say I am going to paste it a few times. One, two, three, four, five, six, seven should be enough. So if I save this and go back into my browser and test it, you can see it's still stretching to 100% of the height.
But when happens when I scroll? Oh, hmm. So the initial column height and the initial body height is the height of the viewport. So when I begin to scroll, that height that was fixed at that size no longer stretches to fit the viewport, so now my column is just sort of overflowing here. So that is really not what I'm wanting. So if I go back into my section, however, and instead of section I say height 100%, what if I said minimum height, meaning, is the shortest it can be as 100%.
So if I save this, and go back in and test it, aha, now it stretches, but unfortunately, the body doesn't. If I go back into the body and tell its minimal height to be 100% as well, trying this method, you're almost always sort of chasing your own tail. Now, the other reason this works--and I am just going to scroll back up at the top and do that again--is because we don't really have any padding or anything like that. If I go back in and reduce this content-- and I am not going to get rid of all of it. I just want to get rid of about half of it. And if I go back up to my section here, and I add say some padding.
So if come in and add padding of 10 pixels. Now, here's the thing about using percentages for both height and width. When you do a padding of 10 pixels, height is added to that. Those effects are cumulative. So the 10 pixels' worth of padding top and 10 pixels' worth of padding on the bottom are added to that 100% height; they're not factored into it. So if I save this, again, test it in my browser, what happens is is even though the content doesn't stretch all the way down, you see we've got a page that can scroll and this is still overlapping, and that's that extra spacing that we are getting through our padding.
So it's really frustrating. Now, we have a really simple layout here. As our layouts get more and more complex, these issues that we are taking a look at right now, they magnify, because you're going to have to start dealing with the height of several elements above this one. So does this mean that you should never try to have elements' content stretch to fit the screen, or its parent? No, that's not what it means at all. But you really need to understand how height works in order to properly plan and execute that type of layout. Typically, for example, there is a lot of wrapping div tags and things like that involved to keep some of those negative side effects from happening.
Now, I recommend checking out a few gallery sites online. A few of my favorites are cssbeauty.com, where you can look at some sample sites that designers have submitted. Pattern Tap, which is awesome. They have different patterns for footers and buttons and sidebars and headers and all sorts of really cool things. There is the bestcssvault.com, which is, again, user-submitted sites. And you could even go to like wordpress.org and check out some of their sample themes. Now, what you're going to find, almost overwhelmingly, is that most designers simply design their sites so that variable-height columns don't negatively impact it.
Sometimes knowing the limitations of a technique is just as important to designing it as knowing its strengths.
- Reviewing the box model
- Calculating em and percentage values
- Controlling how elements display
- Creating fixed, fluid, and responsive layouts
- Structuring content with HTML5
- Floating elements
- Using relative, absolute, or fixed positioning
- Defining column spacing
- Creating grid-based assets and layouts
- Considering mobile-design-specific issues
- Working with multi-column text
- Enhancing page design CSS Sprites