The overflow property says it only applies to block containers, but it should also apply to tables and flex containers.
For non-root elements whose position is 'relative' or 'static', the containing block is defined to be formed by the content edge of the nearest block container ancestor box [or table box], but it should be formed by other types of layout container for certain elements in other layout schemes (for example by the flexbox for flexbox items).
Note that the issue of other layout schemes needing such adjustments to overflow and the containing block hierarchy already exists within CSS21 itself:
Both these issues arose through an oversight when introducing the terms “block container” etc, namely that table boxes were overlooked. This illustrates the special-casing that spec editors must remember to perform in individual layout specs if the problem is not solved more generally.
overflowIn 9.4 (Normal flow), replace:
# Boxes in the normal flow belong to a formatting context, which may # be block or inline, but not both simultaneously. Block-level boxes # participate in a block formatting context. Inline-level boxes # participate in an inline formatting context.
with:
| Boxes in the normal flow belong to a formatting context, which in | CSS21 may be block, inline or table. In future levels of CSS, other | types of formatting context will be introduced. Block-level boxes | participate in a block formatting context. Inline-level boxes | participate in an inline formatting context. Table formatting | contexts are described in the chapter on _tables_.
In 17.4 (Tables in the visual formatting model), replace:
# [...] The table wrapper box establishes a block formatting context. # [...]
with:
| [...] The table wrapper box establishes a block formatting context, | and the table box establishes a table formatting context. [...]
In 17.5 (Visual layout of table contents), replace:
# Internal table elements generate rectangular boxes with content and # borders. Cells have padding as well. Internal table elements do not # have margins.
with:
| Internal table elements generate rectangular boxes which | participate in the table formatting context established by the | table box. These boxes have content and borders, and cells have | padding as well. Internal table elements do not have margins.
In 11.1.1 (Overflow), replace:
# Applies to: block containers
with:
| Applies to: block containers and boxes that establish a formatting context
(This has neither been reviewed nor discussed on the public mailing list.)
In 10.1 (Definition of “containing block”), replace:
# 2. For other elements, if the element's position is 'relative' or # 'static', the containing block is formed by the content edge of # the nearest block container ancestor box.
with:
| 2. For other elements, if the element's position is 'relative' or | 'static', the containing block is formed by the content edge of | the nearest ancestor box that is a block container or which | establishes a formatting context.