Function assert_image_snapshot 

pub fn assert_image_snapshot(
    snapshot: Snapshot,
    rgba: &[u8],
    width: u32,
    height: u32,
)

Available on crate feature snapshot only.

Assert that rgba matches snapshot within DEFAULT_TOLERANCE.

Natively a missing snapshot is written from this frame rather than failed on, so the first run of a new test records its baseline, and SNAPSHOT_UPDATE=1 rewrites one that exists. On the web the baseline is embedded in the binary, so there is nothing to write and a missing one could not have compiled.