Unlike most block-like elements that uses padding box for overflow clip, replaced elements uses content box instead.
Note that replaced elements' content always has the same size as the content box (except embedded SVGs), i.e. inherently can't overflow unless there is rounded corner. They simply ignore CSS overflow property (again, except for embedded SVGs) and always apply a clip when border-radius is present.
Comment 1 by wangxianzhu@chromium.org
, Jun 7 2017