When adding the place-* shorthands, we realized that specifying a fallback alignment for the content distribution keywords (space-around etc.) resulted in parsing ambiguities. We debating using a slash in the shorthand, but decided not to because it would make 2-axis alignment syntax inconsistent with similar syntax elsewhere in CSS, e.g. for scroll-snap-align or background-position.

An alternate idea is to use the slash to separate the fallback from the initial. Imho this also helps make it clearer that we're declaring a fallback alignment, since slashes are used to indicate alternates in general typographic usage.

Examples:

justify-content: space-around / center; /* space-around, fall back to center - both axes */
place-content: space-around / center; /* space-around both axes, fall back to center */
place-content: space-around start / center; /* space-around b-axis, start x-axis, fallback center */

(I think it's probably best to consider this for Level 4, leaving fallback keywords out for Level 3 so we can take Level 3 to CR asap.)

This sounds like the values are basically parallel; why not just use commas to separate the fallback from the normal?

Resolution to defer to L4: https://lists.w3.org/Archives/Public/www-style/2017Apr/0027.html

An alternate idea is to use the slash to separate the fallback from the initial.

I think that would be very confusing because slash is already used in the CSS Grid shorthands (grid, grid-template, grid-area etc) to separate row-axis from column-axis values. A slash in the place- shorthands should have the same meaning, i.e. place-content: <align-value> / <justify-value>.
Examples:

place-content: space-between start / center; /* space-between b-axis with start fallback, center i-axis */
place-content: space-between / center; /* synonym to "space-between center" */

i agree that a slash would be better used as a separator for row-axis and column-axis values. It even makes the shorthand's syntax clearer, IMO

I don't think this syntax issue should be deferred to L4, because adopting the proposal to use / as the separator will be impossible at that point since it wouldn't be backwards compatible. place-content:space-between start currently means align-content: space-between; justify-content: start whereas if we adopt / as the separator and allow for fallback values in the shorthands, then it's a single value and means align-content:space-between start; justify-content:space-between start.

It seems to me that / was actually the preferred separator by most people when the shorthands were discussed in issue #595. Using space as the separator was only accepted under the assumption that fallback values and <overflow-position> would never be allowed in the shorthands. It seems that assumption is now false and I would like to withdraw my "commenter satisfied" for issue #595, fwiw :-).

If you don't want to resolve on an extended syntax here for L3, then I request that these shorthands be deferred wholesale to L4 to avoid creating a situation were the preferred syntax can't be introduced because it would break the web.

No, I was suggesting that the slash separates the initial and the fallback, not the axes. So we'd have align-content: space-between / center for the longhand. This would help make it clearer that center is a secondary value, vs. the current syntax align-content: space-between center.

Then for the shorthand, place-content: space-between / center would set both align-content and justify-content to space-between / center. In cases where you want different values in each axis, we'd follow the border-radius shorthand pattern, e.g. place-content: space-evenly / start center would assign align-content: space-evenly / start; justify-content: space-evenly / center.

(Syntax of border-radius is border-radius: [ <length> | <percentage> ]{1,4} [ / [ <length> | <percentage> ]{1,4} ]?, described in https://www.w3.org/TR/css3-background/#corners, for reference.)

The CSS Working Group just discussed Syntax for fallback alignment in the place-* shorthands, and agreed to the following resolutions:

• RESOLVED: slash must be use to separate main value and fallback value in shorthand and longhands alignment properties