As per this question on stackoverflow the parts of the regex match pattern in the user-defined css attributes (for margin-xxx settings, etc) copied into options doesn’t include the unit specifier ‘mm’, in the function read_options_from_html(html).
Perhaps the ‘mm’ are added as a default in subsequent processing and aren’t needed (I don’t know), however if this is a bug it might explain odd results that occur when any margin attributes are added.