[Alert] Raw implementation with vanilla JS

[JeremiePat]

This is the tracking bug for implementing accessible alert box using only raw web technologies available in browsers.

Link to existing specification or recommendation:

• ARIA Authoring Practices (Alert)

We need to :

• [ ] Document alert boxes requirements and usage (in English)
• [ ] Document alert boxes requirements and usage (in French)
• [ ] Define functional specifications as Gerkins tests
• [ ] Implement a first version of those specification
• [ ] Formally tests those specifications

[JeremiePat]

After some investigation we can define three kind of alert box that have different requirements:

• Inline alert
• Dialog alert
• Notification

This issue intend to handle the first kind of alert, the other two will have their own dedicated issue.