react.dev/cookbook/cb-02-inline-styles.md

id	title	layout	permalink
inline-styles	Inline Styles	docs	inline-styles.html

Problem

You want to apply inline style to an element.

Solution

Instead of writing a string, create an object whose key is the camelCased version of the style name, and whose value is the style's value, in string:

/** @jsx React.DOM */

var divStyle = {
  color: 'white',
  backgroundImage: 'url(' + imgUrl + ')',
  WebkitTransition: 'all' // note the capital 'W' here
};

React.renderComponent(<div style={divStyle}>Hello World!</div>, mountNode);

Discussion

Style keys are camelCased in order to be consistent with accessing the properties using node.style.___ in DOM. This also explains why WebkitTransition has an uppercase "W".