lucy24 - 8:29 am on Sep 17, 2012 (gmt 0)
Where did you learn this?
Have you ever looked at a plain html page with no css but all layers of headers? There are demos floating around. It's actually pretty mind-boggling. Not sure if it's really h3; might be h4 that's 100% font size. All headings are bold and left-justified. h5 and h6 are definitely smaller than regular text. (This does not make sense to me either.)
fwiw, in my boilerplate, h5 is 100% and the others are bigger. h6 is obviously very rare-- I might have it in something like ALL CAPS but then smaller than regular caps so it still "reads" as a header. Er, heading. I have trouble with these words. Rarely are any of my h's either bold or left-aligned, let alone both, unless the page has a wonky semantic structure.
I ran off and did a quick experiment. Browsers are surprisingly consistent-- even MSIE, which has to be at least ten years old-- except that some of them make h6 very small and some make it tiny. Well, maybe I've set a 9-point minimum in some browsers but not all.
<h4> is pretty close to standard bold size. Maybe a hair smaller-- hard to tell, because Palatino bold runs small anyway.
Incidentally, lynx shows <h1> in a distinctive way: Centered, with a different background color. The other five are left-aligned but marked by increasing indents, so <h2> is slap against the edge of the page, <h3> starts only a little outside the edge of the print area, and the others are progressively further in. Say two ems each time.