[css-multicol] Intrinsic Sizing of multicol elements

[rachelandrew]

In the email archive is an issue raised by @fantasai on the 18 March 2015.

There is a bunch of discussion around the issue but I haven't as yet located any resolution or proposed edit. Adding this issue in order that any further discussion can happen here.

[fantasai]

This ended up going into the Sizing L3 spec (and later pushed out to L?), since Multi-col was in CR and we didn't want to add any new requirements for it. For Multi-col Level 1, it should remain undefined. For Level 2 we should make an effort to define this properly, and stealing text from https://drafts.csswg.org/css-sizing-4/#multicol-intrinsic would be a good start.