WIP - Issues from first run through

[textbook]

• Prettier collapses <element></element> to <element/>, e.g. for main and header. Some students were confused by this and not sure how to get back to being able to put children in those elements. Maybe put some content in each one so the page isn't initially blank, too?
• Prettier also adds lots of changes (quotes, line breaks) which makes the commit bigger than it needs to be. We should start from a consistent format (tracking: https://trello.com/c/QP4ZZsgt, and you can configure VSCode to make Prettier use ESLint).