[csswg-drafts] [css-sizing] width: fill needs clarifications

frivoal has just created a new issue for 
https://github.com/w3c/csswg-drafts:

== [css-sizing] width: fill needs clarifications ==
Maybe I'm just dense, but I am afraid I don't quite understand `width:
 fill` (as defined 
[here](https://drafts.csswg.org/css-sizing-3/#valdef-width-fill) ).

Modulo writing modes, what does `width: fill` do that 
`box-sizing:border-box; width: 100%;` doesn't? Assuming there is a 
difference, (and the definitions don't seem completely equivalent, 
even if at a high level they look similar), a note and/or an example 
showing what it's for and how it works would be most welcome.


Please view or discuss this issue at 
https://github.com/w3c/csswg-drafts/issues/317 using your GitHub 
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