You can think about jQuery.cssNumber as a list of all CSS properties you might use without a unit. It's used by .css() to determine if it needs to add px to unitless values.

The keys of the jQuery.cssNumber object are camel-cased and the values are all set to true. If you want to prevent the .css() method from automatically adding the px unit for a specific CSS property, you can add an extra property to the jQuery.cssNumber object.

jQuery.cssNumber.someCSSProp = true;

By default the object contains the following properties:

• zIndex
• fontWeight
• opacity
• zoom
• lineHeight
• widows (added in jQuery 1.6)
• orphans (added in jQuery 1.6)
• fillOpacity (added in jQuery 1.6.2)
• columnCount (added in jQuery 1.9)
• order (added in jQuery 1.10.2)
• flexGrow (added in jQuery 1.11.1)
• flexShrink (added in jQuery 1.11.1)