Webpages are more than simply how they look. By coding with headings, landmarks and semantic HTML, you can expose a rich document outline to users of assistive technologies, as well as search engines.
Nice lesson! Also, it is correct form to have multiple h1's per document, provided that you are using html5 doctype and the h1's are each in a distinct article.
How do you find this extension you are using in the video?
Mark, looks like https://addons.mozilla.org/en-US/firefox/addon/web-developer/ based on her website https://marcysutton.com/web-accessibility-resources