browser bug?

Question

I think case1 is reasonable, but case2 isn't at the point of font-size.
<!-- case1. font size is independent on nesting div tags -->
<div>
<h1>h1 text </h1>
<h2>h2 text </h2>
<div>
<h1>sub h1 text</h1>
<h2>sub h2 text</h2>
</div>
</div>
<!-- case2. font size is dependent on nesting tags -->
<article>
<h1>h1 text </h1>
<h2>h2 text </h2>
<section>
<h1>sub h1 text</h1>
<h2>sub h2 text</h2>
</section>
</article>

Answer 1

Browser depends eventually... of their default css values: some differences occurs, regards of navigator agent, platform and so on.
You can look at projects like reset.css, normalize.css or sanitize.css, which are all attemps with differents techniques for makes browsers render more identical from one to the others ;)