A Brief Survey of webp Support in macOS, 5/29/20:
Answer: Nearly 3 years after the OP's question, it seems the answer remains the same: "webp is not well-supported in macOS".
It appears that after briefly supporting
webp in a beta release of macOS Sierra, Apple decided against it. AFAIK, Apple provided no rationale or explanation for this decision (no surprise there). This may lead some to speculate that it was a petty motivation - as a Google product, perhaps it's the "not invented here" syndrome? I'll stick my neck out, and speculate that Apple will never support webp. Note also that Safari is notably absent from the list of browsers supporting webp.
As @Allan mentioned, Google has provided a webp library, and they have published the sources for their webp library. There is a GitHub repo for webp, but it's an empty facade - perhaps to prevent others from creating a repo under that name?
Google's licensing appears to be non-standard. I won't comment further as I'm a layperson in IP matters.
Google's webp library includes several command-line utilities. It can be installed with MacPorts (
sudo port install webp). For me (my use case), the utilities are useless:
They provide no useful conversions, as they do not decode to the GIF format.
Animated webp files are crudely rendered by
vwebp via OpenGL in one size only.
One of the answers here references the WebPQuickLook repo on GitHub. But as of this writing, this repo appears to be abandoned as there have been no changes to the repo files in 2 years.
The macOS App Store has several utilities that claim some level of support for webp. I've not tried them, but I generally shun the App Store due to several poor experiences.