[css-flexbox-1] 9.2(3)(e) — used flex basis is not an optional substitution

[gitspeaks]

The paragraph at 9.2(3)(e) states:

Otherwise, size the item into the available space using its used flex basis in place of its main size, treating a value of content as max-content.

The phrase "in place of its main size" is confusing and should be removed.

The used flex basis is always either the main size (when flex-basis is initially auto) or a user-declared size that overrides the main size. Therefore, used flex basis is not an alternative to the main size—it's the actual size under consideration.

[Loirooriol]

TBH the entire 9.2(3) step seems to have various problems and probably needs to be rewritten. But I don't get why you dislike a reminder that the preferred main size is ignored in favor of the flex basis?

[gitspeaks]

There’s no “reminder” here—referring to the used flex basis doesn’t imply a specific source for the size. As mentioned, the used flex basis can be derived either from the preferred main size or from the declared flex-basis value.

What you’re describing is an interpretation of “used flex basis” as necessarily referring to a declared value that overrides the preferred main size—but that’s problematic. All the other cases (A–D) are defined in terms of the used flex basis, which can originate from either source.

For example, case A refers to a definite used flex basis but doesn’t specify the source of that definite size—it could just as well come from width: 10px as from flex-basis: 10px.

[Loirooriol]

I don't get what you are saying. Whether the calculation of the used flex basis relied on other properties or not is irrelevant at this point.

[gitspeaks]

“used flex-basis” doesn’t override the “preferred main size”; it reveals or exposes the preferred main size or a user declared flex-basis size. So, saying that the “used flex-basis” is used in place of the main size is a misinterpretation of what a “used value” actually represents.

[gitspeaks]

Simply put, if I tell you that my used flex-basis is 10px, you don't know whether that value comes from width: 10px or flex-basis: 10px. So how can you claim that width: 10px is ignored in favor of flex-basis: 10px?

[Loirooriol]

I still don't see the problem.

[gitspeaks]

I still don't see the problem.

That’s because you’re misinterpreting the text.

You wrote:

But I don't get why you dislike a reminder that the preferred main size is ignored in favor of the flex basis?

But the correct interpretation would be:

But I don't get why you dislike a reminder that the preferred main size is ignored in favor of the used flex basis?

The used flex basis value can very well come from the preferred main size property —so there’s nothing being “ignored in favor of” anything else.

As stated in the spec:

7.2.3. The flex-basis property:

The flex-basis property sets the flex basis. It accepts the same values as the width and height property, plus content.

<'flex-basis'>

auto
When specified on a flex item, the auto keyword retrieves the value of the main size property as the used flex-basis. If that value is itself auto, then the used value is content.