Is a box that establishes a BFC considered a Block Container?

[gitspeaks]

Could you clarify if a box that establishes an independent formatting context is considered a block container? The spec is currently unclear on this point.

[gitspeaks]

For example, a position value of absolute or fixed blockifies a box and forces it to establish an independent formatting context. Is the that box now considered a "Block container" ?

[Loirooriol]

Is a box that establishes a BFC considered a Block Container?

Yes

Could you clarify if a box that establishes an independent formatting context is considered a block container?

Not in general.

a position value of absolute or fixed blockifies a box and forces it to establish an independent formatting context. Is the that box now considered a "Block container" ?

Forcing a box to establish an independent formatting context doesn't turn it into a block container. Either it was already a block container and then it will establish a BFC, or it was something else and will establish a different kind of formatting context, or "establish an independent formatting context" doesn't apply.

[gitspeaks]

But setting position: absolute forces the principal box to establish an independent formatting context, and we know that an absolutely positioned box establishes a BFC. Therefore, setting position: absolute makes the principal box a 'block container' regardless of its display status.

[Loirooriol]

we know that an absolutely positioned box establishes a BFC

False, it can establish other kind of formatting contexts

setting position: absolute makes the principal box a 'block container' regardless of its display status

False, it can be another kind of container

[gitspeaks]

According to the spec

Floats, absolutely positioned elements, block containers (such as inline-blocks, table-cells, and table-captions) that are not block boxes, and block boxes with 'overflow' other than 'visible' (except when that value has been propagated to the viewport) establish new block formatting contexts for their contents.

[Loirooriol]

CSS2 is outdated. As I pointed out in #11126 (comment)

[Loirooriol]

In case you missed the big, red, bold warning:

[gitspeaks]

So the note in the new spec:

"A position value of absolute or fixed blockifies the box, causes float to compute to none, and forces the box to establish an independent formatting context."

This means that:

1. If the box has inner display: flow, flow-root, or table, it will establish a new BFC once it is positioned.
2. If the box has inner display: flex, it will establish a new Flex Formatting Context once positioned.
3. If the box has inner display: grid, it will establish a new Grid Formatting Context once positioned.
4. If the box has inner display: ruby, it will establish a new Ruby Formatting Context once positioned.

[gitspeaks]

Can the same be said on flex item ?

1. If the flex item box has inner display: flow, flow-root, or table, it will establish a new BFC for its contents.
2. If the flex item box has inner display: flex, it will establish a new Flex Formatting Context for its contents.
3. If the flex item box has inner display: grid, it will establish a new Grid Formatting Context for its contents.
4. If the flex item box has inner display: ruby, it will establish a new Ruby Formatting Context for its contents.

[Loirooriol]

Basically, yes. If it's not replaced or some special HTML thing or such.

Also, about ruby, once blockified the principal box is a block container (see #9265) which will then establish a BFC, the ruby formatting context is established by a non-principal inline-level ruby container.