If I understand correctly, you want the user to select double-speed in their browser? YouTube will then speed it up - but for me in Firefox anyway, it does so while keeping pitch constant. You want to slow the sound to half speed, also
keeping pitch constant, yes? This is a process with unavoidable artifacts in both directions - usually audible as clicks, echoes or a hollow sound. If you could allow the pitch to fall and rise, the fidelity would be much better, as only simple resampling is needed, but I don't think YouTube lets you change speed without pitch correction.