I use headings in answers all the time. These are the rules I (try) to follow:

• Avoid h1 headings (single hash prefix). You almost never do need this. Remember, there's already one h1 at the top of the page - it's the title of the question!

• Don't use headings at all unless there are at least a few paragraphs between them. In many cases, what you really want is... A list!

• Do use headings when there are multiple, discrete topics being explored in a single answer. I've seen people write hard-to-read walls of text in cases where just a few clear, descriptive headings would make it much easier to recognize the presence of individual sections within the text. Use headings to give an at-a-glance view of the structure of your answer, so that folks can skip to (or skip back to) the relevant section quickly.

If the SE devs didn't want us to use Markdown headers at all, they'd just disable the feature.

Then again, just because something is technically allowed doesn't meant it's always a good idea.

Certainly, if you feel your post actually benefits from the structure provided by multiple levels of headings, you should use them. However, using large headings just to grab attention can be seen as distracting and obnoxious behavior.

In particular, if your post needs only one type of headings, please consider using the smallest kind that still stand out from body text (typically ###), or even just bold or italic text.

I wouldn't personally edit you post merely over such a minor issue as the difference between second and third level headings, but neither do the oversized (and seemingly gratuitous) headings make me super-inclined to vote it up, either.

Heading 1

Heading 2

Heading 3

Heading 4

Heading 5

Heading 6

Bold text

Text

I think SO just has a styling issue here: Heading 3 (###) is just bold text with a different font (which actually makes it appear smaller than plain bold text). Heading 2 (##) on the other hand is already much larger and has a weird over-large margin below it. Heading 1 (#) again is only a bit larger.

Suggestion: Fix margin of h2, make h3 larger. Or h2 and h1 smaller.

Keep accessibility in mind when formatting web content

Use formatting for its intended purpose. Please don't use it gratuitously! Screen readers provide structure and verbal cues for users who can't see the larger text size, different font, etc.

Headers

Sighted user get visual cues to help them organize information on a webpage: font size, placement, bolding, color, etc. Users of screen readers rely on programmatic headers <h1> to <h6> (Markdown # to ######) to convey structure.

WCAG 2.1 has guidelines on Using h1-h6 to identify headings

1. Check that heading markup is used when content is a heading and the heading markup indicates the appropriate heading level for the content.
2. Check that heading markup is not used when content is not a heading.

Point 1. above means that text should not be marked bold, italicized, etc., to convey content structure. Point 2. above means that headings should not be used to convey emphasis, mood, or any other text formatting.

Text formatting

Bold text indicates importance. Italics indicate emphasis in another voice or mood. Sighted users get different visual indicators for text formatting like these. Users of screen readers rely on programmatic text formatting for verbal cues of their special status.

The WCAG 2.1 has guidelines on Using semantic markup to mark emphasized or special text.

Personally, I would rather see fewer headings in answers. It's becoming quite a trend on The Workplace with

Executive Summary

blah blah

Background Reasoning

more stuff

and so on for several screens. I generally feel if your answer is so long and complex that it needs to be broken into sections, the question is too broad or your answer is too long or both. I would not miss this markup if it was removed.

ES IST NOCH KÄSEBROT DA!

Headlines are an essential part of an answer if you need to get the message out or divide content.

Therefore I'd say it's accepted practice to use them sparely and distinctively if they serve a purpose, like my beautiful headline above.

Which actually is shouting as all uppercase, so the comment by that guy was a bit wrong, not bold, but uppercase text is normally considered as shouting, or in case of my headline, just a cheering expression of joy.

This is perhaps an example what should be considered not so good:

╔═══════════════════════════════╗

║ ♥ thank you for reading ... ♥ ║

╚═══════════════════════════════╝

Old question that got dusted off today when I linked to it in this MSO question.

I really like Shog9's answer, but I go one step further, personally, in my use of headings.

I believe that, for answers, the least obtrusive use of headings is the best. By that, I mean that I use the smallest (highest numbered) headings possible for the structure of my answer.

So if I only have one level of heading to separate multiple sections of an answer, I use H4 (since I didn't realize until today that H5 was an option, and it's really only the same thing as adding a "bold" tag anyway):

Section heading

Some paragraph content.

Some more paragraph content.

Etc.

Next section

So on

Side Note: And apologies for the semantically incorrect use of blockquotes to indent. I spent 15+ minutes trying to get this example to indent properly with hard spaces and non-breaking spaces, but Stack has a formatting issue (it seems) when using Headings with non-breaking spaces.

In the extremely rare case that I might need two levels, I would add the H3 as the top level. But I would try to flatten out the structure first.

I understand that this "breaks the normal rules" when it comes to, say, always starting with an H1, followed by an H2, etc. But those rules are meant for the HTML page itself. Answers are not an HTML page. They are database content that gets inserted into an HTML page.

That database content, and the users who create them, are not expected to have any knowledge of the page into which they will be inserted. In a typical Stack question/answer, the current page structure is that the question title itself is formatted H1. So that technically means (if you take the W3C recommendations literally and try to apply them to Markdown) that any answer headings should start with H2.

But:

1. How is the typical Stack user supposed to know this? Are they supposed to examine the page markup before using headings in their answers? (Rhetorical question - No.)
2. There is no future guarantee that an additional heading level might be added by Stack, or even removed.

So no, IMHO Markdown formatting should not, and cannot, follow the "HTML Rules" for the use of headings.

It would be great if (pie in the sky) Stack could adjust the Markdown headings (and their formatting) to the semantic structure of the page itself, but this honestly seems like overkill.