This task covers setting up the foundation for our new custom element. We will start by implementing the base or "Informational" variant of the message bar only. At this point our message bar will just support displaying an icon and some message text - it will not yet be dismissible, or have support for buttons or links.

To get started we can run the addwidget scaffold command:

./mach addwidget moz-message-bar

This will create all the necessary files for the component. In addition to implementing the markup and styles for our basic element, we should also add a story and at least one simple test case.

The Credential Management team reacently added a Lit-based login-alert component that looks similar to what we're envisioning for our message-bar component. When getting started it may be helpful to adapt that code by moving it to toolkit/widgets.

https://hg.mozilla.org/mozilla-central/rev/587915c7b1a7