Having a good night’s sleep helped me realize my mistake — I was confusing a containing block for a box. Sorry for the trouble. Re-reading the definitions of containing blocks in [css-display](https://drafts.csswg.org/css-display-3/#containing-block) and [css-2](https://www.w3.org/TR/CSS2/visudet.html#containing-block-details) snapped me out of it. A containing block is a *rectangle* with a size. It may be established by different things, such as a grid area of a grid container, or a content-box of an ancestor box. So to answer my own question: no, box-sizing should not be considered, because it doesn’t apply to the concept of containing block. Instead, the size of the containing block depends on the layout context in which a box is to be laid out, and padding percentage values should resolve against that. -- GitHub Notification of comment by andyjakubowski Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/6509#issuecomment-897440718 using your GitHub account -- Sent via github-notify-ml as configured in https://github.com/w3c/github-notify-ml-configReceived on Thursday, 12 August 2021 08:17:11 UTC
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