Ask about the actual configuration
A media type can include a container and codec information. A broad video/mp4 question supplies less detail than a configuration naming the actual codecs. Two resources with the same extension can contain different media and produce different outcomes. Preserve the exact queried configuration when interpreting a result.
MDN documents empty, maybe and probably results for canPlayType. MediaSource has a separate support query for its own path. These results should not be collapsed into a universal yes for all resources using a similar file suffix. A negative prerequisite and a successful full session are different kinds of evidence.
Build a useful decision chain
Imagine a player offered two authorised representations. It first checks whether the required API exists, then queries the declared type, then attempts the selected media. A probably result allows investigation to proceed; it does not remove the need to handle download, parsing and decode errors.
If one representation fails after a positive prediction, record where the failure occurred. A segment can be malformed, an initialisation resource can be absent or access can expire. Those observations do not necessarily contradict a type-support prediction because the query did not inspect every byte or authorise the account.
Label the evidence accurately
A compatibility table based on documentation or API predictions should say so. Mark a real playback result separately with the browser version, media configuration and test conditions. Do not label every device using the same browser brand as verified because one configuration returned probably on one machine.
For viewers, the practical question is whether the service documents the intended player route and whether the actual session succeeds. A positive technical predicate cannot establish catalogue availability, distribution permission or a subscription entitlement. Keep those business facts outside the browser support result and retain an ordinary supported fallback when the preferred representation cannot start.
Questions about this guide
Does probably mean the programme was tested?
No. It is a prediction about a declared type.
Do container support and actual playback prove the same thing?
No. Actual playback also depends on media details, access and delivery.