<!DOCTYPE html> <style> .ok { color: red; } #-- { color: green; } #-foo { color: green } #--foo { color: green; } .-- { color: green; } .-foo { color: green; } .--foo { color: green; } .fail { color: green; } #- { color: red; } .- { color: red; } </style> <div class="ok" id="--">This should be green.</div> <div class="ok" id="-foo">This should be green.</div> <div class="ok" id="--foo">This should be green.</div> <div class="ok --">This should be green.</div> <div class="ok -foo">This should be green.</div> <div class="ok --foo">This should be green.</div> <div class="fail" id="-">This should be green.</div> <div class="fail -">This should be green.</div>