A comment on use cases that was brought up over here.
There's a use case that is not addressed by XBL1 and impossible to quite address in a JS library that goes as follows. Say you have some data in a <table>. Semantically it's a data table. But you may want to present it as a graphic (SVG or canvas).
XBL1 will let you construct a block or inline box for the <table> and then stick a shadow tree containing just an <svg> or <canvas> inside. But it would be nicer if that wrapper block/inline box were not there and if the box that were created were just the <svg> or <canvas> box... So effectively, treating the element the component is attached to as a <content> element in terms of box tree construction.
Is this a use case we want to address? -Boris
