https://codecov.io/gh/substack/tape/branch/master/graph/badge.svg works, but https://codecov.io/gh/substack/tape/branch/HEAD/graph/badge.svg does not. “HEAD” redirects to the default branch everywhere on github.com, and it’d be really nice not to have the codecov badge be the only place the default branch name is hardcoded.
(see Add coverage badge by thomasrockhu · Pull Request #544 · ljharb/tape · GitHub)