Alert banners are used to communicate an urgent message or a call to action.

Use colors from the web color palette, which have enough contrast ratio to meet Web Content Accessibility Guidelines. The alert banner shouldn't take over the browser window or prevent users from completing other tasks.  

One of these two designs is recommended.

Top of the Page

Screenshot of a banner at the top of an web page.

Under the Horizontal Navigation

Screenshot of an alert banner under the top navigation on an web page.