tedster - 6:38 pm on Sep 17, 2012 (gmt 0)
Not a lot bigger, but it HAS to be bigger.
I'd say the H2 has to be more prominent in some way - but it can be the same size. I've been involved with an online magazine for about ten years. The H2 elements in the current template are the same size as the body text, but bold and a gentle blue color. That's a stylistic choice that complements the look and feel of the page. But the H2 elements ARE subheads in the article. It's that logic that matters most, IMO.
In fact, I've seen H2 tags used as a "run-on head", where even the line break at the end is eliminated via CSS. That still can be legitimate.
It really helps to understand a bit of the history of HTML. It starts with GML (Generalized Mark-up Language) which was developed by IBM in the 1960s - and GML already contained H tags! So H tags have both an academic and a functional purpose that is deeply embedded in electronic documents long before Sergay and Larry were born.
Used properly H tags can contain useful relevance signals. Abused (as they often have been online) they add no value to a document and search engines don't trust them. I remember back in 2002 hearing from a Google search engineer that they couldn't even trust H1 tags as much as they trusted body copy!