[realPubkey]

Most of the time you should not even check for the runtime. Instead you should check for the specific feature support that you want to use.

[c0balt]

+1, though sometimes it's useful to know the runtime variant & version to implement workarounds for implementation quirks or polyfills.

[dspillett]

Or to aid recreating problems were, for instance, you've accidentally relied on undefined behaviour that happens to be consistent in most places.