Are `system-font` atomic or not?

[svgeesus]

Spun out from

• [css-fonts-4] Which type of font family names are system font names? #9292

In the section on system fonts we read:

System fonts can only be set as a whole; that is, the font family, size, weight, style, etc. are all set at the same time.

which is seemingly contradicted by

These values can then be altered individually if desired.

I think what this means is that the system font name can only be set using the font shorthand, but once set then things like font-weight etc can be changed. Right?

[Loirooriol]

Yes. In Blink, system fonts behave somewhat like var(): they set all the longhands to a magical internal value that won't be resolved until computed-value time. But you can still use the cascade to change individual longhands as you please.
I think other browsers are similar.

[cdoublev]

I think other browsers are similar.

FF seems to reset reset-only subproperties instead of setting them to a magical internal value.

el.style.font = 'caption'
el.style.fontVariantPosition; // 'normal' in FF / empty string in Chrome

[emilio]

We have a magic internal value as well, just not for all the font longhands, only the ones we actually use, see bug 1797146.

As of right now, that's only font-family, font-size, font-style, font-stretch, and font-weight.

[svgeesus]

Can we capture this magic into the spec, assuming we think the magic is needed and correct?