This PR adds interactive example for crossorigin attribute. I chose to show how it works with script element, because showing how tainted canvas work would require long and complex JS code, while crossorigin in <link> effect can be seen mostly in network devtools tab.

Currently my private host is used to store cross-origin JS code, so this PR shouldn't be merged. I won't be able to store it indefinitely, so we need server owned by Mozilla with Access-Control-Allow-Origin: * HTTP header which can store following JS file:

setTimeout(() => {
    throw new Error('Secret message of error inside cross-origin script.');
}, 500);