Venn Diagram entirely in CSS

A friend of mine alerted me this weekend to just how much I have a weird fascination with Venn diagrams. I decided to roll with it. So yeah, I have an irrational love of Venn diagrams. But that begs the question, can I make a Venn diagram with just CSS?

I found a couple of examples out there:

• Dusting off front-end skills: CSS Venn Diagram
• HTML And CSS Venn Diagram

But I felt like they had a bit too much fluff in the HTML markup. Not that there is anything technically wrong with their implementations. I prefer complexity in my CSS and not in my HTML. It's probably just a subjective thing, but I do.

So how do you do it?

First you create 3 divs. 1 for each Venn circle, and 1 for the overlap section. Each div contains a p with content in it.

<div id="venn">
    <div><p>People Who like Venn Diagrams</p></div>
    <div><p>People who read my blog articles</p></div>
    <div id="overlap"><p>Just me.</p></div>
</div>

Then you go to style each of the circles. Give them matching heights and widths, and a border radius of half of the height. This creates the circle. Then give each one an opacity below 1. This will ensure that when they overlap they will form a new color.

#venn div{
    height: 360px;
    width: 360px;
    border-radius:180px;
    -khtml-border-radius:180px;
    -moz-border-radius:180px;
    -webkit-border-radius:180px;
    border: 1px solid #000;
    display: table;
    float: left;
    opacity: .6;
}

I then created two rules based on the nth child css selector to color each of the circles. I also padded to ensure that there would be a space to write in the overlap section.

#venn div:nth-child(1){
    background-color: #FF0000;
    color: #FFF;
    padding-right: 60px;
}

#venn div:nth-child(2){
    background-color: #0000FF;
    color: #FFF;
    margin-left: -60px;
    padding-left: 60px;
}

Finally I styled the overlap section using relative positioning and pulled it back towards the center.

#venn #overlap{
    border: 0;
    height: 30px;
    width: 30px;
    left: -350px;
    top: 150px;
    color: #ccc;
    position: relative;
    text-align: center;
    z-index: 10;
    opacity: 1;
}

The real trick is to watch the pixel counts because a couple are directly related.

To create a circle:

• width must equal height
• border radius must equal 50% of width.

To overlap circles:

• Circle 2 must have negative x left margin
• (Or Circle 1 must have negative x right margin)
• Each circle must have x padding-left or x padding-right to ensure its text doesn't spill over borders

It looks like the example works across modern browsers, including IE 9, but not previous versions.

Source: http://www.terrenceryan.com/blog/post.cfm/venn-diagram-entirely-in-css