[css-values] Rename `inherit()` to `parent()`, add more `inherit`-like variant as `inherit()`

[Crissov]

Comments tend to be overlooked after an issue has been closed, like #2864 was. Therefore, reraising as a separate topic.

The inherit() function should behave more like the inherit global keywords in that it always refers to the same (custom, computed) property it is used within! That means, drop the first parameter, thus only expect an optional fallback value within the functional notation.

For accessing ancestral values from arbitrary properties, there should be a separate function, called something like parent() or parental().

[Loirooriol]

I argued for inherit() because it's a natural extension. But fair, I guess there can be various natural extensions:

• Inherit from another property (instead of the same property)
• Inherit from another element (instead of the parent, originating element, etc.)
• Inherit with a fallback value (to be used instead of the inherited value if it's the guaranteed-invalid value)
• ...?

Do we want all of these, and possibly combinations thereof?

[Crissov]

Sure, my point is that usually, where keyword and function with the same name exist, they yield (almost) the same result when the function is called without arguments.

[dbaron]

I don't think parental() makes sense from an English usage perspective. We don't tend to use it when we talk about tree structures in computer algorithms; it's really much more specific to talking about family structure/responsibilities.

[Crissov]

Point taken re parental(). I’m fine with parent() or even parent-value() and possibly alternative suggestions.

I also want to point out again that MathML has already introduced the function add() to CSS, which is now widely supported. It is currently limited to the math-depth property and an integer argument, but could probably be generalized to all numeric property values.

[Loirooriol]

my point is that usually, where keyword and function with the same name exist, they yield (almost) the same result when the function is called without arguments.

This is an argument for making the <custom-property-name> optional. Not for renaming IMO.

[Crissov]

I thought it would be a problem if the first parameter also became optional, but you never need a fallback value if the property specified in the first parameter (or implied by its omission) cannot be void (i.e. guaranteed invalid) like all standard properties.

a {
--custom: green;
  background: var(--custom);
  border-color: lime;
  color: olive;
}
a>b {
--custom: orange;
  border-color:
    currentcolor
    inherit(/* border-color */) /* like `inherit` */
    inherit(background-color) /* future level */
    inherit(--custom, red)
  ; /* = olive lime green green */
}

That means, a unified inherit() could be possible.

[fantasai]

I don't think that the use case for inheriting the value of the corresponding property while providing only a fallback is strong enough to warrant a dedicated syntax. In the rare case that the author really wants this, they can specify the name of the property explicitly. The ability to specify which property to inherit from is useful, on the other hand, and the name is pretty straightforward about what it does - it gets the inherited value from the named property.

[Crissov]

@fantasai Sorry, I failed to properly state my updated proposal for inherit() before, which requires a change to the current syntax in Level 5:

- <inherit()> = inherit( <custom-property-name>, <declaration-value>? )
+ <inherit()> = inherit( [ <custom-property-name>, <declaration-value>? ]? )

In Level 5,

• if a parameter is provided it must be a custom property name.
• a second, optional parameter takes the fallback value if that custom property has the guaranteed invalid value.
• if all parameters are omitted, the inherit() function shall return the value of the current (longhand) property, so it can be used among other values in a single declaration – unlike its global keyword sibling, inherit.

In Level 6 or later,

• the first parameter might instead accept any standard property name.
• the second parameter would be useless for properties that exists, but it could apply for all other <ident>s.