I’m not ruling out the possibility of the translation changing the structure. For example, French can express emphasis through words, and may not need italicisation (eg. C’est moi qui l’ai fait! rather than <em>I</em> did it!) My experience is, however, that this is not very common – particularly when dealing with non-literary translations.

But I think that even if you wanted to change the element structure, you’d be better off working with semantic markup than presentational markup.

I like how you have defined <em> and <strong>, and to a point I like the explanations of <i> and <b>. But they are still presented as somewhat vague presentational devices in that they can cover a range of semantics: <i> is suggested for things “such as a taxonomic designation, a technical term, an idiomatic phrase from another language, a thought, a ship name, or some other prose” and the examples suggest Latin names for flora and fauna and dream sequences; <b> is suggested for “key words in a document abstract, product names in a review, or other spans of text” and the example include special eye-catchers and lede sentences.

And then follows the phrase “whose typical typographic presentation is boldened/italicized”.

I’m trying to make the points that

1. during localisation, people may want to separate the presentational styling for the suggested uses where more than one appears in a localised version of a document (ie. not all italicised or boldened)
2. such things may not be typically boldened/italicised in other scripts, ie. that may not be the best default in Chinese/Japanese

Therefore…

The spec says that “The i/b element should be used as a last resort when no other element is more appropriate.”, but I think it implies that once you have exhausted the elements offered by HTML5, you have run out of options. You haven’t. You could use class names to label things.

So I would suggest an alternative wording along the lines of “The i/b element should only be used as a last resort when no other element is available and you want the text to be visually distinct in the absence of or inability to use a style sheet. This should only be used as a fallback device, however. It is much better to use an i/b element with a class name that describes the intent of the text, and associate that where possible with a rule in a style sheet.”

For example, I would prefer the spec to change its examples slightly like this:

<p>The <i class="taxonomy">felis silvestris catus</i> is cute.</p>
<p>The term <i class="term">prose content</i> is defined above.</p>
<p>There is a certain <i class="foriegn" lang="fr">je ne sais quoi</i> in the air.</p>

If one document contained all these examples, I would now be free to restyle them individually and separately as I wish, without having to trawl through the HTML to make the changes.

But I should probably now send this to the HTML5 list for discussion.