[css3-page] Proposal: Making variable page area widths in a document ?optional

Hi,

By using :left, :right, or :first pseudo-classes, it is currently possible
to make
left, right, or first pages have different page area widths.

In the following example, right pages are 10cm narrower than left pages:
@page :left {margin-left: 3cm;margin-right: 4cm;}
@page :right {margin-left: 9cm;margin-right: 8cm;}

I propose that supporting such variable page area widths should be optional.
(Supporting variable margins must be mandatory as far as the page area width
remains the same.)

Pros:
- Rendering engine implementation will be simplified.
    WebKit (and I believe many other rendering engines) assumes that page
    width doesn't change throughout a document. Changing that assumption
    will impose fundamental implementation change to the rendering engine(s).

Cons:
- Freedom of changing page widths within a document is lost.

In my opinion, the Pros far outweighs the Cons. The rendering engine
complication risk is real, while the benefit of changing page widths
within a document is hypothetical -- I've never read a book that has
changing page widths.

Yuzo