Character Class Spacing: the text-spacing property

The following comments have been moved from #6950

I see a couple of things that could be provided for users if autospace were a property rather than being just one of the many values in the text-spacing property, and in fact when trying to describe how to manage inline spacing in Japanese i actually found it intuitive and simpler to handle autospacing separately from other text-spacing values (see https://r12a.github.io/scripts/jpan/#letterspace ff.). I'll note also that many years ago when CSS did define an autospace feature and IE implemented it, it was defined as a property, afair.

The following practical use cases seem hard to support if autospace is a value of text-spacing.

[1] It doesn't appear to be (certainly easily, but maybe not at all) possible to have, for example, autospacing set for the document as a whole (by default or by a style declaration), and then turn it off for some parts of the content.

[2] Given that the gap used for autospacing is usually smaller than the width of a space character, it may be desirable to apply autospacing to content that has been written where space characters have been used to separate alphanum and han text in order to reduce the size of the gaps. This can also be useful to standardise the layout if space characters have been used in some parts of the text and not others. For dealing with such content it would probably be useful to have a feature that replaces the space characters with an autospace gap.

[3] The size of the gap used for autospacing can vary, depending on the preference of the publisher. There needs to be a way of specifying the width of the gap to be used.

It seems to me that having an autospace property with values such as <width>|none|replace-space would make it easier to handle these use cases, and to boot be more intuitive for content authors.