In JSX all DOM properties and attributes should be camelCased to be consistent with standard JavaScript style. This can be a possible source of error if you are used to write plain HTML.
The following patterns are considered warnings:
var React = require('react');
var Hello = <div class="hello">Hello World</div>;
The following patterns are not considered warnings:
var React = require('react');
var Hello = <div className="hello">Hello World</div>;
If you are not using JSX you can disable this rule.