When specifying a pixel value for your inline style
prop, React automatically appends the string "px" for you after your number value, so this works:
var divStyle = {height: 10}; // rendered as "height:10px"
ReactDOM.render(<div style={divStyle}>Hello World!</div>, mountNode);
See Inline Styles for more info.
Sometimes you do want to keep the CSS properties unitless. Here's a list of properties that won't get the automatic "px" suffix:
animationIterationCount
boxFlex
boxFlexGroup
boxOrdinalGroup
columnCount
fillOpacity
flex
flexGrow
flexPositive
flexShrink
flexNegative
flexOrder
fontWeight
lineClamp
lineHeight
opacity
order
orphans
stopOpacity
strokeDashoffset
strokeOpacity
strokeWidth
tabSize
widows
zIndex
zoom