The baseline behaviour of text-spacing should always turn on ideograph-alpha and ideograph-numeric when in the horizontal writing direction. In the vertical direction, it should be turned on when the western text is rotated¹ and turned off if the western text is not².

Microsoft Word has ideograph-alpha/ideograph-numeric turned on by default. The iOS UI also has this spacing, but I'm not sure if it's done on the rendering level, or spaces are injected during string interpolation.

On other platforms, it is not supported out of the box. Authors end up adding spaces themselves inconsistently, leading to horrible UIs like this:

CSS should support this turned on as the default.

There may also need to be an additional property for authors to specify the width of the inserted space they want. The default 1/4 space is fine.