To check if a reference space is supported for a given reference space type type and XRSession session, run the following steps:

1. If type is not contained in session’s XR device's list of enabled features for mode return false.

I found some conflict between the spec explainer and the xrSession_requestReferenceSpace_features.html WPT testcase.

• According to the spec session’s XR device's list of enabled features has to be used to check if a reference space is supported for a session
• According to the testcase: Reference spaces that aren't default features are rejected when not requested as a feature, even if they have been added before to device's list of enabled features.

IIUC device's list of enabled features can contain accumulated features from multiple requestSession calls, and the spec would need to be rephrased to:

To check if a reference space is supported for a given reference space type type and XRSession session, run the following steps:

1. If type is not contained in session’s list of requested features for mode return false.