The text in Firefox doesn't lose subpixel antialiasing, at least in my browser.

Also, it's perfectly possible to use GPU compositing with subpixel antialiased texture, you just need a separate texture to store the (Ra, Ga, Ba) pre-component "alpha" vales and either dual source blending, two rendering passes (first to apply alpha to destination, second to add color), three rendering passes (one per color), or a pixel shader that takes the pre-blend target surface as an input.

If your browser doesn't do that, file a bug.