[Quoth]

No, the <em>, <strong>, <s>, <del>, and <ins> are pure web browser driven specs and absolutely not needed for accessibility. I compared with human read text.

A publisher can't know what the writer's intention was, any more than an HMTL tweaker of ebooks, and I proved it doesn't matter, because the <i> and <b> work fine. The accessibility issues are elsewhere. You've swallowed propaganda. The Browser heads deprecated <i> and <b> for <em> and <strong> and then un-deprecated them and saved face by writing the difference is semantics.

These are not real publishing standards or real accessibility. Putting meaningful alt text for images is a real thing.

<s> vs <del> isn't semantics. The <s> should never be used, but CSS decoration and <del> and <ins> are not semantic variations, they are specialist use.

Underline should only be CSS decoration and is obsolete, replaced by bold in style guides. Should be reserved for links, where it's automatically rendered anyway.

It's not about what percent of people use accessibility. The important thing is to concentrate on real accessibility issues. The HTML standards people are not going to get any consistent real semantic use of <strong> and <em>. Anytime I've seen them on web pages or ebooks there is no semantic justification, they are simply being used as aliases to <b> and <i>. Real semantic use of <em> would be rare and the publisher isn't going to decide on a word for word basis ever. They simply use entirely <strong> and <em> or entirely <b> and <i>.