Good morning awp,
From KDP Guidelines:
3.10.2 HTML Guideline #2: Anchors Must Be Added Before Formatting Tags
Correct: <a name=”Chapter1”/><h1>Chapter 1</h1>
Incorrect: <h1><a name=”Chapter1”/>Chapter 1</h1>
(By name= I suppose they wanted to say id=)
When atlantis generates the epub, it does the following:
<h2 class="h21"><a id="ncx1"/>Hyperlinks</h2>
If you use this same HTML to be used for the mobi, it violates this rule.
Now, it may work as it is but I want KDP to be happy with me, so I wisely follow their rules.
Now, let's assume that some of us use Sigil after Atlantis and for any reason we have to recreate the TOC, Sigil will add its own id:
<h2 class="h21" id="sigil_toc_id_1"><a id="ncx1"/>Hyperlinks</h2>
So ... unless Sigil causes urticaria to Atlantis, I would suggest that it creates:
<h2 class="h21" id="ncx1"><Hyperlinks</h2>
And everybody will be happy.