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https://html.spec.whatwg.org/multipage/image-maps.html#image-map-processing-model
> For historical reasons, the coordinates must be interpreted relative to the displayed image after any stretching caused by the CSS 'width' and 'height' properties (or, for non-CSS browsers, the image element's width and height attributes — CSS browsers map those attributes to the aforementioned CSS properties).

ぐえー。