WebM on Mac OS X

Perian, a QuickTime plugin should support WebM since version 1.2.2 according to their website.

Not really much to choose from on macOS. While Perian is indeed VP8-capable, to play modern VP9 WebM's you may try minimalistic mpv player.

On the second thought, you may not need a dedicated video player for any WebM at all, just open the file with any recent version of Firefox, Chrome, Opera or Safari and watch the video.