<em> should be treated as progressive enhancement
Published on in Accessibility
Screen readers don't announce emphasized text, so your text should be understandable even without the emphases.
Martin Underhill writes that bold and italics aren't read by screen readers.
Interestingly, the NVDA screen reader added support for emphasis [in 2015], only to remove it following complaints by users.
I had noticed before that screen readers indeed don't announce if the currently read text is bold or italics.
But I hadn't considered this mindset:
Text-level semantics like italics and bold should be treated as a progressive enhancement. In other words, your sentences should make sense without emphasis; those
<strong>wrappers should just offer a nice added extra for [sighted users].
Strikethrough is a similar but more difficult case because it adds content. Read Martin's another post Be careful with strikethrough for more details. The post contains several good points why strikethrough is better avoided entirely.