Style SVG circle with CSS

As per the SVG 1.1 specification you can't style the r attribute of an SVG circle using CSS https://www.w3.org/TR/SVG/styling.html#SVGStylingProperties. But you can do:

<circle cx="168" cy="179" r="59"
        fill="white" stroke="black"
        onmouseover="evt.target.setAttribute('r', '72');"
        onmouseout="evt.target.setAttribute('r', '59');"/>

In SVG 2, which is partially supported by some modern browsers, you can style the r attribute of circles using CSS. https://www.w3.org/TR/SVG2/styling.html#PresentationAttributes

It can be done in CSS(3), by setting the transform-origin of the circle to its center and then using scale transformation:

circle {
  transform-origin: center center;
}

circle:hover {
  stroke-width: 10px;
  transform:scale(1.2, 1.2);
}

Want to only use CSS? Use line instead of circle.

<svg xmlns="http://www.w3.org/2000/svg" version="1.1">
    <style>
    .circle {
        stroke-width: 59;
        stroke-linecap: round;
    }
    .circle:hover {
        stroke-width: 71;
    }
    </style>
    <line x1="168" y1="179" x2="168" y2="179" stroke="white" class="circle" />
</svg>

http://jsfiddle.net/rLk7rd8b/