I discovered most of these issues while testing on IE8 with IE7 compatibility mode on.
To address these, for both the html and xhtml exporters we've adjusted the line-height from '1' to 'normal'; and now the 'line-height' attribute is set on the span that contains the text with font-size, for proper calculation.
Also, 'Proportional' line spacing for the html exporter now behaves like the one in the xhtml exporter: it uses a float number instead of percentage.
You need to check out the jasperreports repository for these changes.