There should be three emphasis elements:
<em> Increases emphatic semantics by one level. *No* default rendering style for visual media, default rendering for other media not specified.
<i> Equivalent semantics to <em>. Default rendering style for visual media is a language-dependant alternative glyph set of the same font family and weight (e.g. italic/курсив, oblique, カタカナ). Default rendering style for other media not specified (at least the same as <em>).
<b> Equivalent semantics to <em>. Default rendering style for visual media is the same font family and glyph collection, but higher weight. Default rendering style for other media not specified (at least the same as <em>, perhaps louder for aural).
The <strong> element is deprecated, replaced by nested levels of <em> or it's visual-specific variants.
Thus where visual presentation is important, <i> and <b> can be used semantically (they are equivalent) and <em> ignored. Where visual presentation is not important, <em> can be used without concern for what <i> should sound like. The basic point is that <em> has no default rendering style, discouraging it's misuse for "i want italic text and people tell me <i> is bad these days, so i'll use <em>".
- Nicholas.
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