Used to load class name mappings and other information related to CSS modules.
When an app implements the .CssModule interface, it MUST return a response
file with the following attributes set:
classNames string: a JSON string containing an object that maps original
class names to namespaced counterparts.
Example value: {"button":"NF45D5AD8-button"}
fontNames string: : a JSON string containing an object that maps original
font names to namespaced counterparts.
Example value: {"MyFont":"NF45D5AD8-MyFont"}
animationNames string: : a JSON string containing an object that maps
original animation names to namespaced counterparts.
Example value: {"bounce":"NF45D5AD8-bounce"}
version string: Used for caching .Request calls.
A unique string generated from calling the .Request method
on the same file. The .Request method may in fact not be called, but it
MUST be guaranteed that the value of the version attribute will change
every time the response to the .Request call changes.
head string: A HTML string referencing dependencies that is to be injected
in the <head> section of the response.
Also, the app must respond to .Request calls according to the above rules.
For example, the css-2 app
implements the .CssModule interface.