[css-writing-modes] Behavior of <iframe> in vertical writing mode

I'm wondering how <iframe>s should be handled when writing-mode is vertical.

First, what should the default size be, in the absence of any other 
constraints? In horizontal mode, we have 300px (w) by 150px (h), from 
the rules in CSS 2.1 [1,2]. Should it remain the same in vertical mode, 
or should width and height be exchanged? The description of Abstract Box 
Layout in [3] would seem to imply the latter. (Gecko currently does 
this, but it seems Blink does not.)

And second, should the writing mode of the <iframe> element be applied 
to its contents? I'd have assumed "no" (and AFAICT no browser currently 
does this), but [4] says

# The content of replaced elements do not rotate due to the writing mode:
# images, for example, remain upright. However replaced content 
involving text
# (such as MathML content or form elements) should match the replaced 
element’s
# writing mode and line orientation if the UA supports such a vertical 
writing
# mode for the replaced content.

which could easily be read as being applicable to <iframe> content, 
unless that content explicitly resets writing-mode to its initial value. 
Clarification?

Thanks,

JK

[1] http://www.w3.org/TR/CSS2/visudet.html#inline-replaced-width
[2] http://www.w3.org/TR/CSS2/visudet.html#inline-replaced-height
[3] https://drafts.csswg.org/css-writing-modes-3/#abstract-layout
[4] https://drafts.csswg.org/css-writing-modes-3/#block-flow