[css-backgrounds-3] Can `background` syntax be made more future-proof?

[cdoublev]

I hope this editorial change can be applied to <final-bg-layer>:

background = <bg-layer>#? , <final-bg-layer>

- <final-bg-layer> = <bg-image> || <bg-position> [/ <bg-size>]? || <repeat-style> || <attachment> || <visual-box> || <visual-box> || <'background-color'>`
+ <final-bg-layer> = <'background-image'> || <'background-position'> [/ <'background-size'>]? || <'background-repeat'> || <'background-attachment'> || <'background-origin'> || <'background-clip'> || <'background-color'>`

Basically the same change (omitted for brevity) would apply to <bg-layer>.

Why?

(1) background-clip is defined with <bg-clip>, which produces <visual-box> | [ border-area || text ], but <*-bg-layer> only accepts <visual-box>. By using property value ranges (<'background-*>) instead of specific types (<bg-*>), the background syntax can be decoupled from longhand or type definitions in specific level(s) of the spec.

(2) CSS Backgrounds editors could extend background-repeat without changing <repeat-style> (which is used in mask syntax) or creating <bg-repeat>. The same applies to background-attachment and <attachment>, which is however not used elsewhere.

(3) <final-bg-layer> mixes <'background-color'> and <bg-*>, which seems inconsitent.

(4) ... || <'background-origin'> || <'background-clip'> || ... helps explaining in which positions are expected the corresponding longhand values.

Optionally, <bg-image>, <bg-position>, <bg-size>, could be dropped.

^{(edit) removed specific references to CSS Backgrounds 3 and 4 after d45bed7.}

[AtkinsSJ]

It's not quite that simple unfortunately, because those longhand properties take a comma-separated list of values, which isn't allowed inside a background layer. (Swapping <bg-image> to <'background-image'> changes it to <bg-image>#.)

<'background-color'> is the exception because it doesn't have multiple layers.

[cdoublev]

Somewhat counterintuitively, # is ignored when referenced by a property value range production:

Additionally, if the property’s value grammar is a comma-separated repetition, the corresponding type does not include the top-level comma-separated list multiplier.

https://drafts.csswg.org/css-values-4/#component-types

[AtkinsSJ]

Oh that makes things more convenient here!