mathiasbynens/mothereffingcssescapes — comment on commit ec51aab2b013058aaf58996f21a0982e19fc16e5
File: eff.js line 45
By mathiasbynens:

This serves as a fast path for the case where the ID/classname consists entirely of hyphen-minus characters. It could be removed and everything would still work, but it would be slightly slower for this exact case.