The HTML Standard contains an algorithm to compute heading levels and has for the past fifteen years or so, that’s fairly complex and not implemented anywhere. E.g., for the following fragment
<body>
<h4>Apples</h4>
<p>Apples are fruit.</p>
<section>
<div>
<h2>Taste</h2>
<p>They taste lovely.</p>
</div>
<h6>Sweet</h6>
<p>Red apples are sweeter than green ones.</p>
<h1>Color</h1>
<p>Apples come in various colors.</p>
</section>
</body>the headings would be “Apples” (level 1), “Taste” (level 2), “Sweet” (level 3), “Color” (level 2). Determining the level of any given heading requires traversing through its previous siblings and their descendants, its parent and the previous siblings and descendants of that, et cetera. That is too much complexity and optimizing it with caches is evidently not deemed worth it for such a simple feature.
However, throwing out the entire feature and requiring everyone to use h1 through h6 forever, adjusting them accordingly based on the document they end up in, is not very appealing to me. So I’ve been trying to come up with an alternative algorithm that would allow folks to use h1 with sectioning elements exclusively while giving assistive technology the right information (default styling of h1 is already adjusted based on nesting depth).
The simpler algorithm only looks at ancestors for a given heading and effectively only does so for h1 (unless you use hgroup). This leaves the above example in the weird state it is in in today’s browsers, except that the h1 (“Color”) would become level 2. It does so to minimally impact existing documents which would usually use h1 only as a top-level element or per the somewhat-erroneous recommendation of the HTML Standard use it everywhere, but in that case it would dramatically improve the outcome.
I’m hopeful we can have a prototype of this in Firefox soon and eventually supplement it with a :heading/:heading(…) pseudo-class to provide additional benefits to folks to level headings correctly. Standards-wise much of this is being sorted in whatwg/html #3499 and various issues linked from there.