impress.js | presentation framework based on the power of CSS3 transforms | Runtime Evironment library

Classes declared in CSS can be used (and probably are) with JS events. The body's classname is changed dynamically by JS.

The default class has no special styles, it's the same as the class attribute won't be there.

and how do these body.xxx work?
It works the same way as any class on other HTML elements.

addEventListener does not overwrite previously added event listeners, this is one of its advantages over using the on* event properties / attributes.

In order to do what you want you are going to need to add your own impress:init event listener, before the one added by impress, do the same setup as the library does, changing what needs changed, and then calling stopImmediatePropagation().

stopImmediatePropagation will prevent any other listeners of the same type from being executed, meaning impress's impress:init event listener will not be triggered.

There are 2 possible ways to do:

1. Store the individual answers in JS and check the value of selected radio against the those answers.
2. Store the answers of questions in a database and make a server request to validate user inputs against correct answers.

Latter is a recommended solution as it'll keep your answers invisible to the users but if it doesn't matter you can use the former method.

Possible solution via Method 1: