[css-values] toggle() for shorthands is not forward compatible

https://drafts.csswg.org/css-values/#funcdef-toggle

If toggle() is used on a shorthand property, it sets each of its longhands to a toggle() value with arguments corresponding to what the longhand would have received had each of the original toggle() arguments been the sole value of the shorthand.

Let's consider display: toggle(block, flow-root, inline).

Right now, display is a longhand property, so we have:

• block flow -> block flow-root
• block flow-root -> inline flow
• anything else -> block flow

But it seems likely that we will want to convert display into a shorthand. Then, the example will become

display-outside: toggle(block, block, inline);
display-inside: toggle(flow, flow-root, flow);

Due to short-circuiting, that's equivalent to

display-outside: block;
display-inside: toggle(flow, flow-root);

Which behaves like

• block flow -> block flow-root
• anything else -> block flow

Start with the toggle() definition in the linked CSS Values specification section and compare its shorthand-property wording with the display example in this issue. Done means the specification resolves the forward-compatibility concern and defines behavior that remains consistent if display becomes a shorthand.