[css] CSS Circle with border

You are missing the border width and the border style properties in the Border shorthand property :

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.circle {
    border: 2px solid red;
    background-color: #FFFFFF;
    height: 100px;
    border-radius:50%;
    width: 100px;
}
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<div class="circle"></div>
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Also, You can use percentages for the border-radius property so that the value isn't dependent of the circle width/height. That is why I used 50% for border-radius (more info on border-radius in pixels and percent).

Side note : In your example, you didn't specify the border-radius property without vendor prefixes whitch you propably don't need as only browsers before chrome 4 safari 4 and Firefox 3.6 use them (see canIuse).