See this twitter conversation: https://twitter.com/lajava77/status/849182533791227904

TL;DR: Safari and Chrome ship justify-content: space-evenly, but it only works in grid, not in flexbox.

In their implementations, in flexbox, it falls back to the same behavior as flex-start. But since it parses fine, you cannot use the cascade or @support to provide a different fallback.

The real solution is for both browsers to fix this by completing their implementation, but my question is:

• should they unship justify-content: space-evenly until then (even though it works fine in grid)?
• If no, as a stopgap measure until full implementation, should they make it behave like justify-content: space-around (most similar) or justify-content: center (specified as the fallback alignment, for different situations)?

Should the spec say something about this particular kind of partial implementations, besides what's already in https://drafts.csswg.org/css-align/#conform-partial

@mrego @javifernandez @tabatkins @fantasai