I'd like to investigate other formatting options here. In the first case at least, I'm not sure the boldness is helpful.

That's a good point; the typographic choice here may benefit from a review, and in that case we can discard this PR.

At any rate, the format should be abstracted into what it really means – such an abstraction should eventually be a part of this PR.

Do we have any further plans here? If not, could the present PR be applied as a small-increment improvement?

Updated this to use the b series rather than the non-existing bx series.

Reverted to \textbf. The substitution from bx to b is a "silent substitution" that is WAI, and similar silent substitutions happen elsewhere in the document (e.g whenever there is code in a heading).

Please implement a macro that gives this construct some semantic meaning.

Actually, I don't think the bolding helps at all. The goal of marking the code as a "placeholder" is served perfectly well with a plain italic. I think with this change it's simple enough and we can keep the \textit around as is.

I also found another place were it could be applied.

I agree with @vermiculus that we should have a macro for this kind of thing; a quick search for other bitmask types shows that they use a bare \textit{...}, which is what our \placeholder{...} expands to. That's covered by at least one of #553, #326, and #319 =)

Let's get this change in to fix the presentation and leave the cleanup to the other issues.

If there's interest, I'm happy to make a change that uses \placeholder. But we should be sure that we want that particular macro, or otherwise devise a new one first.

I'm a pretty deft TeX programmer. Tell me the syntax you want and I'll be happy to implement it in that other issue (can it be referenced here?) 😄

What distribution of TeX Live is being used to compile this? (Use $ tex --version to find out.)

We should also consider other fonts we can use. If you don't already know, all fonts are now fair game.

@vermiculus: I think we can probably use \placeholder, as zygoloid suggested. It already exists and seems to have the appropriate semantics.

Depending on context we may also like a version of it that disables italic correction, but that's a minor point.

Ah, I didn't realize such a macro already existed 😄

Depending on context we may also like a version of it that disables italic correction, but that's a minor point.

Adding a variant to \placeholder is easy enough (take a look at xparse documentation), but any such markup should have a specific purpose. My question to you would be 'depending on what context?', but that might be a better question to answer off-line or on some instant media. Feel free to email me directly.