[tdeck]

It's because some browsers close the currently open heading tag when they encounter a new one (since these tags can't be nested) and others do not. Since the text size is applied as an H3 style and is relative to the parent, the nesting of these tags (or not) determines whether the effect of the style is cumulative.

[gpvos]

If I understand the other arguments correctly, IE also saw them as nested, but defined the text size of h[1-6] elements as absolute, not relative.