Sigil's ToC generator uses the <h> tags to create the ToC.
There are some
shady-unwashed-peasant type publishers that use <p> or <div> tags for headings - even though the heading tag is designed for...headings... - I say they should all get 50 lashes with a wet noodle!!
You can use CSS to style - make them look like - anything you want. The same CSS will make p/div/h tags look the same, so there really is no reason to NOT use the semantically correct tag...in fact, using the correct tag allows reading devices and editing software to work properly... as you found out with the ToC generators.
eg.
Code:
CSS:
h3 {font-size:1.2em; text-align:center; font-weight:bold}
h3 span {display:block; font-size:1.1em; font-weight:normal; font-variant:small-caps; margin:1em auto}
HTML:
<h3>Chapter 4 <span>The Way it Should Be</span></h3>
is much better than
Code:
CSS:
p.I_want_this_to_look_like_a_header
{font-size:1.2em; text-align:center; font-weight:bold}
p.This_is_the_chapter_sub-title
{font-size:1.1em; text-align:center; font-weight:normal; font-variant:small-caps; margin:1em 0}
HTML:
<p class="I_want_this_to_look_like_a_header">Chapter 4</p>
<p class="This_is_the_chapter_sub-title">The Way it Should not Be</p>