fn calculate_box_size(
target: &Element,
observed_box: &ResizeObserverBoxOptions,
) -> Rect<f64>Expand description
https://drafts.csswg.org/resize-observer/#calculate-box-size
The dimensions of the returned Rect depend on the type of box being observed.
For ResizeObserverBoxOptions::Content_box and ResizeObserverBoxOptions::Border_box,
the values will be in px. For ResizeObserverBoxOptions::Device_pixel_content_box they
will be in integral device pixels.