The CSSStyleDeclaration returned by getComputedStyle always returns a fixed value for length, ignoring the presence of any custom properties, while the CSSStyleDeclaration for inline style correctly counts custom properties. Similarly, item() on the style from getComputedStyle never returns a custom property.

Firefox includes custom properties on computed style's length, and returns then as the last values for item().

http://jsbin.com/rurahoziya/1/edit?html,js,console,output