What’s the opposite of a nbsp?

You want the unicode character ZERO-WIDTH SPACE (\u200B).

You can get it in HTML with ​ or ​.

Explicit breaks and non-breaks:

LB7 : Do not break before spaces or zero width space.
LB8 : Break before any character following a zero-width space, even if one or more spaces intervene.
http://unicode.org/reports/tr14/