Items are automatically added to your horizontal navigation bar as you create top-level pages. Unlike a vertical navigation bar, there are limitations to the number of top-level links you can include ...

Open your HTML file in a text editor. Ensure that the divider for the navigation bar, identified with a tag, is located directly following the divider for the header.

If you wanted a tiered second navigation that is also horizontal you can use this feature and add a few CSS tweaks.