Empty Container elements usually occur when a container element such as a list was selected in the widget, but was never filled with any content. They can also be inadvertently inserted when content is copied and pasted in. Because empty containers are not usually visible on the page, they are not readily apparent until you look at the html code of the content.
The Siteimprove page report for this issue will look similar to the one in the image below. We can see that the content view of the report is highlighting an empty container in red near the Ethnic Studies content. You can also toggle to the html view of
the report to identify where the issue is in the code itself.
In the image below we have located the widget in Sitefinity and clicked on the html view in the upper right. Using ctrl+f to search the content, we have located the content which includes the word "ethnic" which was where the siteimprove page report indicated the empty header container was. We can see in the html that the container is an empty unordered list (<ul>). To resolve this issue we can simply delete this empty container element.