[Jellby]

Originally Posted by Tex2002ans

So is this because there is no such thing as a font with "smallcaps + bold" (just like there are fonts for "bold", "italics", "bold/italics", "smallcaps")?

Depends on the font, but there certainly exist bold smallcaps fonts (and italic smallcaps, although that's something LaTeX does not like)

What/how is Sigil (or a browser) displaying such a thing?

I guess they mostly fake the smallcaps with the bold font. You can try it here.

Maybe you haven't noticed it, because faked smallcaps look thinner than they should, which is somehow countered with a bold font... but with the uppercase letters or with real smallcaps it is quite noticeable.

I will take a closer look and do some testing later today. So what you are saying is implementing your .xpgt and doing something like:

<p class="noindent"><spanclass="ade">I<span class="simsmallcaps">NTERNATIONAL PROBLEMS</span></span><span class="noade smallcaps">International problems</span>, economic as well as political, [...]

Seems like it might get hairy real quickly (even moreso than just leaving the simulated smallcaps in there by themselves).

Indeed, that would be quite a nuisance... No, what I suggest is

<p class="noindent"><span class="smallcaps">International problems</span>, economic as well as political, [...]

span.smallcaps {
font-variant: small-caps;
}

and then in the .xpgt, which is only visible to ADE, add:

<ade:template xmlns="http://www.w3.org/1999/xhtml" xmlns:ade="http://ns.adobe.com/2006/ade" xmlns:fo="http://www.w3.org/1999/XSL/Format">
  <ade:style>
    <ade:styling-rule selector="span.caps" font-weight="bold"/>
  </ade:style>
</ade:template>

which is the equivalent of:

span.smallcaps {
font-weight: bold;
}

(or underlined, or grey background, or whatever style we want to replace the smallcaps with in ADE).