There are some cases where the computed style for an element that breaks between regions to be different in each region. For example: - percentage margins/paddings - they are computed using the width of the containing block, that may vary between regions - region styling for layout properties - it is possible to style differently the margins, paddings etc of an element based on the region where it gets laid out. Also, region styling line height properties changes the meaning of EM units for all the other Length properties - region styling for paint properties - the content needs to use the paint styles of the region it is getting painted into Use this meta bug to track all the issues related to style diffs between regions.