#7945 Need to distinguish section-header ids from sitewide ids
If you have a section ### foo then it'll be rendered as id="foo" and get CSS applied if we have a #foo rule.
We could add a prefix like "sec-foo" or something with a custom slugify method. See [6cc40c] for an example of that (we ended up not using that commit). However, this would break existing links that have an anchor target in the URL.