The steps

  1. Go to Appearance → Editor and choose Patterns, then the Header template part (or Footer).
  2. Select the spot where the strip should sit, press + and search for ribbonfly.
  3. Pick a Text ticker for an announcement, or a Logo strip for a row of press logos.
  4. Write the content, adjust the sidebar settings and press Save.

The change applies to every template that includes that part. If your theme has more than one header, edit each one.

Why this is better than a site-wide plugin

Some marquee plugins add a bar to the top of every page from a settings screen. It works, but it also runs on every page, whether you wanted it there or not, and it is yours to switch off in a screen you will have forgotten. A block in a template part is visible in the editor, belongs to a template you can see, and is removed by deleting it.

Keeping it quiet

A strip in the header is on every page, which is exactly why it should be easy to stop. The Pause button stays visible beside the strip, hovering or tabbing into it pauses it, and a visitor who has asked their device for reduced motion sees it standing still with the strip scrollable by hand.

Wide and full width

The block supports wide and full alignment, so in a theme that allows it a header strip can span the viewport while the rest of the header stays in the content column.

For a banner with a message and a link, the announcement bar guide walks through one.