[vthriller]

> em dashes are, in most locales, not to be surrounded by a space

This is definitely not the case for at least French and Russian, which means markup renderers now have to guess text language or force authors to declare such in some metadata header. And it gets even more complicated with inclusion of block quotes in different languages.

[chrismorgan]

It’s not hard and doesn’t need language awareness; I described how to detect it: if there’s no space before an end-of-line em dash, suppress the segment-break-replacing space.

[tczMUFlmoNk]

There seem to be some locales or styles that use asymmetric spacing. From the Zen of Python—note different spacing based on context and position within the sentence:

    There should be one-- and preferably only one --obvious way to do it.
    Although that way may not be obvious at first unless you're Dutch.
    [...]
    Namespaces are one honking great idea -- let's do more of those!