Comment 2 for bug 573304

“There is no consensus as to whether a space should be included between the number and percent sign in English. Style guides – such as the The Chicago Manual of Style – commonly prescribe to write the number and percent sign without any space in between.[3] The International System of Units and the ISO 31-0 standard, on the other hand, require a space.[4][5]” – http://en.wikipedia.org/wiki/Percentage

To me, using the space looks consistent with the way a space is added in front of all the other units.