We return "failure" from a bunch of algorithms, but I don't think we define what we mean. (It'd be a singleton type like undefined and null.) Should we add a definition in https://infra.spec.whatwg.org/#primitive-data-types?

@inexorabletash also points out that we're missing <dfn> markers for null, true, and false, and we're missing an Infra-level "undefined" entirely.