For those wondering why: subpixel rendering requires knowing the subpixel layout of the display. It’s something your OS knows, but your web browser doesn’t tell the website.
Also for those wondering how you can get subpixel AA on your canvases anyway: if you use a classic 2d rendering context (not WebGL), you can use canvas.getContext("2d", {alpha: false}) to create a canvas with a fully opaque background, which will also cause fillText to render with subpixel AA on most browsers.