Here is how an H2 Headline looks on this blog currently, without any additional special CSS that you would expect for special things like post titles.

This is how an H3 Headline currently looks on this blog

There is a slight difference but very small.

However I have made a tough decision today

Stop Using H2 For Subheadlines Within Posts

At least only use them for very short headlines for a specific emphasis.

Why H2 Headlines are Bad

A huge and growing proportion of blog readers are subscribers, and their first impression of each piece of your content is within their feed reader.

How your content is displayed is in the hands of the feedreader developers.

First of all I should note that this does not affect post titles. They are universally handled in a different way.

Here is what I saw today that helped me

Here is what H2 titles can look like in the Feedburner management screen

Here is what H3 titles can look like in the Feedburner management screen

Now it should be noted that view of my RSS feed is never seen by the public. However I have seen many sites that use H2 headings that large.

Feed Readers

Let’s take a look at how this looks in Google reader

H2 Headlines in Google Reader

H3 Headlines in Google Reader

H2 Headlines in Pageflakes

H3 Headlines in Pageflakes

Pageflakes handle H2 by adding a page wide underscore and additional space under the headline rather than a massive increase in display size compared to H3.

Lets take a look at H2 in Bloglines

Here is Bloglines H3 Headline

It should be noted that Bloglines alternates backgrounds for the posts.

Bloglines differentiates H2 and H3 using colour more than size, though H2 is slightly larger. It should also be noted that H3 in Bloglines looks exactly the same as a post title.

A reader of Bloglines would typically focus on post headlines more than content headlines, thus it is my opinion it is better to have subheadings that look like post titles.

Want to be sneaky? I haven’t tested this but if you make every subheading a clickable link back to your original post you might have more click-troughs. To achieve this you would have to work out your post URL in advance. Hmm, another idea for my plugin list.

Email

Lots of people subscribe to blogs by email. In fact on many blogs this may be as many as 50% of subscribers, so it is important how your blog is presented.

Lets take a look at Gmail (after being sent by Feedburner)

This is a H2 heading

This is a h3 headline

Feedburner in their email preferences does give you specific control of post headlines and body text. They do not give you control of other elements such as heading tags.

My feeling is that with a basic H2 tag in gmail, a long sub-headline is going to look very messy.

Retro fixed formatting?

It would be possible to use fixed font sizes for all your content, but in my opinion that is a backwards step.

Conclusion

It is important to think about how your content will look where your reader will read it.

It is my belief that H2 is more likely to be defined with an excessively large font than H3 if your content is picked up for syndication on another site.

Current feed reader CSS settings are such that unless you apply a custom CSS modification to your browser for a particular site, H2 could be ugly.
In the case of Bloglines, there could be an advantage using H3 hyperlinks for all your subheadings, if you could work out post URLs before you publish.

Things like heading sizes in the content you read are subjective, but based on my own research as above, I am going to switch to using H3 headlines for most of my blog content.