Opening a new issue to track a follow up discussion of a previous one.

In #6950 (comment), @MurakamiShinyu wrote:

The text-spacing: normal definition is now:

normal -- Specifies the baseline behavior, equivalent to space-start trim-end trim-adjacent ideograph-alpha ideograph-numeric.

I am a little confused about the "baseline behavior.

Before ideograph-alpha and ideograph-numeric were added to normal definition, the meaning was clear to me, that is, the each value included in the definition of normal is the default value, so when we specify the text-spacing property partially changed from normal we can specify just changed values and omit values that are included in the normal definition.
For example, when just text-spacing: trim-start is specified, it implies omitted normal values, trim-end and trim-adjacent. i.e., text-spacing: trim-start was equivalent to text-spacing: trim-start trim-end trim-adjacent.

However, after ideograph-alpha and ideograph-numeric were added to normal definition, it is no longer that simple. Is text-spacing: trim-start equivalent to text-spacing: trim-start trim-end trim-adjacent ideograph-alpha ideograph-numeric? That will be useful when we want to change only trim-start/space-start/space-first behavior and keep other normal behavior, but if so, how do we change just ideograph-alpha/ideograph-numeric behavior from normal?

Then I decided to interpret it as follows:

• text-spacing: trim-start is still equivalent to text-spacing: trim-start trim-end trim-adjacent. The omitted default *-start/first, *-end, and *-adjacent are kept but ideograph-alpha and ideograph-numeric are turned off.
• ideograph-alpha and ideograph-numeric are turned on only when these values are explicitly specified, or normal (or auto, probably) is specified.

(I implemented it in Vivliostyle.js as per this interpretation. vivliostyle/vivliostyle.js#1080)

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However this spec is not very clear and not very useful.

It would be better adding a new keyword, no-interscript-space, to turn off the interscript spacing.

The ideograph-alpha || ideograph-numeric part of the value definition becomes [no-interscript-space | ideograph-alpha || ideograph-numeric] and we can specify only this part to the text-spacing property when keeping other default (normal) behavior.

He then followed up with:

Or maybe it would be better to change the ideograph-alpha || ideograph-numeric value to ideograph-alpha <number> || ideograph-numeric <number> where the <number> represents the space width as a multiple of the fullwidth advance measure. The normal definition would contain ideograph-alpha 0.125 ideograph-numeric 0.125. Examples:

• text-spacing: ideograph-alpha 0 ideograph-numeric 0 turns off the ideograph-alpha/ideograph-numeric spacing and keeps other normal behavior.
• text-spacing: trim-start only changes the *-start/first part of normal behavior. i.e, is equivalent to text-spacing: trim-start trim-end trim-adjacent ideograph-alpha 0.125 ideograph-numeric 0.125.