Ah, you're right, my translation of espace fine to En was incorrect, thanks for the correction.
An intriguing problem in typography is the patterns that spaces can form between words on different lines (called rivers); in traditional typography it is checked for but I don't know of any rendering engine that would do that automatically (in a browser, or on an e-reader for example).
I believe that both TeX (Knuth-Plass Line Breaking Algorithm) and InDesign (paragraph composer – expired US Patent 6,510,441) do this, so there is at least one open-source implementation that could be used as a starting point. Unfortunately though, the awareness for good and bad typography seems to be so low that this is probably not a priority for browser and e-reader vendors. It would be wonderful to have this as part of WeasyPrint or something similar.
TeX doesn't do this automatically, but is rare to see "rivers" in TeX produced publications due to the superiority of its paragraph building algorithm.
An intriguing problem in typography is the patterns that spaces can form between words on different lines (called rivers); in traditional typography it is checked for but I don't know of any rendering engine that would do that automatically (in a browser, or on an e-reader for example).