Sure, but once the text is cached in a layer that layer might be rotated or scaled or similar, in which case subpixel antialiased text would cause ugly artifacts. Keeping subpixel antialiased text in the layer and re-rendering the layer without subpixel antialiasing when a rotating or scaling animation starts would lead to stutters in the animation.
Looking forward to high DPI displays obsoleting subpixel antialiasing as it leads to a bunch of problems when wanting to create fluidly animating UIs.
I don't know what Firefox does on GNU/Linux or OSX but on Windows 7 with DirectWrite, applying transformations to subpixel-AA'ed text is very fast and has excellent fidelity.
Yeah, but in the example the text is not transformed, it's merely overlaid on a composited surface.
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.