This is a bit of a wild guess since I’ve done very little with publishing paths downstream from Paratext, but in general, alternating with NBSP is a useful trick any time you want to display a sequence of multiple spaces in an HTML view. This includes some non-website applications; e.g. Paratext and Paratext Lite both use HTML views to display Scripture text.
Be aware that, if you’re editing in Paratext, it proactively removes extra spaces, so a sequence of EN EM EN spaces gets reduced to a single EM space when typed or pasted into Paratext.
If you instead alternate with NBSP (which Paratext represents as a tilde, to make it visible when editing), you can preserve whatever is on either side of the NBSP. For example, if you paste in a sequence of EN NBSP EM, Paratext does preserve that.
Using a zero-width joiner may work even better than NBSP, if you want more precise control over the total spacing or if you’re entering this in Paratext and don’t want to see the tildes. (I’m guessing this sequence is actually something you’d insert downstream from PT with a script or regex.)
Again, this is guesswork. Hopefully, someone in publishing can chime in if either of these is a good approach, or if there’s a better way.