It also affects floats for which the block container is a containing block by: - Discarding the block-start margin of any float whose block-start outer edge coincides with the block-start inner edge of the container. - Discarding the inline-start/inline-end margin of an inline-start/inline-end float (or equivalent) whose outer edge on that side coincides with the inner edge of the container. - Zeroing the inline-axis margins of a float for the purpose of calculating its intrinsic size contributions or its size in the containerâÂÂs inline axis. - Trimming the block-end margins of a float to the extent necessary to prevent such a margin from extending the block size of a block formatting context root. https://w3c.github.io/csswg-drafts/css-box-4/#margin-trim-block