[fill-stroke] Proper "fill" property value definition to replace "<'background'> with modifications"?

The value of the fill property is currently defined as "<'background'> with modifications". On top of not describing the actual modifications, in all other property definitions, this value needs to be interpreted with the value definition syntax, and a literal interpretation would consider that with and modifications are keywords, not regular spec prose, in other words that one should expect values that include "with modifications", as in:

div {
  fill: #F00 with modifications;
}

Could the spec define the value more precisely with actual grammar? Looking at the definition of <'background'>, I suppose the intent is to transpose it to the different fill painting properties. Trying to run that exercise, I wonder whether the property value could rather be defined as [<fill-layer># ,]? <final-fill-layer> with the following definitions:

<fill-layer> = <paint> || <position> [ / <bg-size> ]? || <repeat-style> || <attachment> || <box> || <box>
<final-fill-layer> =  <'fill'> || <paint> || <position> [ / <bg-size> ]? || <repeat-style> || <attachment> || <box> || <box>

(via https://github.com/w3c/webref/issues/127#issuecomment-1129954792)

Start with the fill shorthand definition in the CSS Fill and Stroke specification and compare it with the referenced background value definition and CSS value definition syntax. Review the proposed fill-layer and final-fill-layer grammar, then confirm that the specification defines the intended values without treating prose as keywords.