Monday, June 30, 2008

On the Importance of Clearing Floats

I've been doing a lot of work with the Big Three of web development -- HTML, CSS and JavaScript. Prior to Yahoo!, I had done a lot of work with Bindows, a powerful "do everything" JavaScript framework. You built your entire web application with Bindows. You never manipulated HTML or CSS directly. It was all abstracted for you with their API.

I loved working with Bindows. It allowed me to improve my JavaScript skills and to build some really cool prototypes. Unfortunately, one of the niceties of Bindows -- that of abstraction and protecting you from the details of HTML and CSS -- turned out to be a real detriment for me.

My HTML and CSS skills suffered, but more importantly, to really fine tune the layout, you have to have access to the underlying structure and presentation layer.

Even some of the most basic and important concepts such as the importance of using and clearing floats weren't etched into my brain until I stepped foot at Yahoo!. I had read about it, understood enough to recognize it ( at least pass the interview ), but it wasn't until I really started doing hardcore development that I realized that to control layout means to control how things flow on the page and that meant understanding how to float and clear elements.

In fact, understanding floats is probably THE most important thing that you need to know when using HTML and CSS. Arguably, there are a bunch of other things that are also important such as knowing how to invoke hasLayout for IE, but I've found that a lot of the layout problems can be fixed by knowing when to clear floats.

When you float an element, you position an element to the left or right of another element. It's a very effective and fundamental way to position elements. Here's an example.

We're floating only two elements. The first is the entire "Left Side" div. It's floated to the left of the "Right Side" div. The second, is the image of Cola, the Yorkshire Terrier. That's floated to the left of the descriptive text. Everything is nicely formatted in Firefox, Safari, Opera and IE7 ( I haven't tried IE6, but it should work there as well ).

If you use Firebug, you'll see that we "clear floats" in three places --
  • At the container level -- this is the div with class "container" that contains the "Left Side" and the "Right Side" divs
  • The "Right Side" div -- this is the div with the class "right-stories"
  • The list element found inside the "Right Side" div
When we clear floats we put elements back into the flow so that they don't flow outside of the layout. There are many ways to clear floats, but the easiest way that I've found is to set the CSS property overflow to hidden. A good writeup of that technique is found here. It's found near the end.

So using Firebug, look for "overflow:hidden" and disable all of it. The elements overrun their borders and resembles a jumbled mish-mash of elements and seem to defy the box model. Here it is.

Whenever you observe strange layout like that, make sure that you're clearing your floats. Remember, floated elements come out of the flow so adjacent or containing elements need to set overflow so that all elements fall back into position. Think of setting overflow:hidden as getting all the misbehaving elements back into line.

Note that you can observe this visually, but you can also use Firebug and inspect the element. Hover over the element and if that element overlays another, that element is out of the flow. If you can identify and fix float issues, you'll be King for a Day!

Have fun!

1 comment:

skypoet said...

The astute reader will notice that I added IE6/IE7's propreitary zoom property to the "good" layout. This was needed to trigger layout ( hasLayout ) in IE6.

hasLayout is one of those mystical and magical properties that makes layout work in IE6.

A good read of what it does is here.