Reported by Anton Prowse
The WG resolved to accept a proposal for Bug 18346 that contains the following wording:
# Certain values of the 'display' property cause an element of
# the source document to generate a *principal box* that contains
# descendant boxes and generated content and is also the box involved
# in any positioning scheme.
but a desire was expressed to be more specific about which values of 'display' result in a principal box.
Firstly, do inline elements generate a principal box (which may well be fragmented across multiple line boxes)? Is that consistent with the inline formatting model defined in CSS21, and if not, is it possible to change that model without breaking implementations?
Secondly, will other values of 'display' defined in future specifications necessarily result in a principal box? (It is not strictly necessary to address this for CSS21 errata, but it may provide useful input.)
Boris Zbarsky provides the following case that would need to be accounted for when trying to spec how an inline element generates a principal box:
ע<span style="border:1px solid green;background:yellow">ִבְרִית 17</span>95