As can be seen on http://www.spi-inc.org/donations/ there is no difference in size for HTML H2 and H3 elements. Both use a 19.2 pixels font size by default. Both also have CSS font-weight of 700. There is a difference between the two in that level 3 is italicized, making it possible for people used to the site tell which is which, but newcomers looking at a level 3 section heading cannot tell from style alone that the section is part of its level 2 section.
This probably affects all spi-inc.org pages, but there are apparently few pages using level 3 sections.
HTML allows 6 levels of sections, making it a bit hard to allow distinguising all easily using size alone without making level 1 titles huge, so it is a good idea to keep using styling other than size in addition.
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Philippe Cloutier
http://www.philippecloutier.com