As you know, GoogleBot uses Chrome version 41 to crawl the web, which means it doesn't see everything the latest version of Chrome sees when crawling the web.
Ashley Berman Hale posted on Twitter that John Mueller from Google said at SMX Munich that it will be a while before that gets updated. He said maybe by the end of this year or early next year. John said it requires a big update and that takes time.
Here is Ashley's tweet:
Looking for Google to update rendering from Chrome v.41 to the latest version? @JohnMu estimates later this year or early next year. It's a big project.
— Ashley (@BermanHale) March 20, 2018
wah.#SMX #smxmunich
So we got some time until Google can see CSS custom properties and other things like that.
Forum discussion at Twitter.
