[css-page] What default does `size` descriptor have when portrait/landscape is not specified?

[nt1m]

When <page-size> is not specified, this case is defined in the spec:

If a is not specified, the size of the page sheet is chosen by the UA.

However, if you write something like size: letter, the specification does not say what the behavior should be:

https://drafts.csswg.org/css-page/#page-size-prop

Should it be portrait or UA-chosen as well?

Right now, one WPT expects it to be portrait (see https://github.com/web-platform-tests/wpt/blob/master/css/css-page/parsing/size-valid.html#L67 ) because it expects letter portrait to be the same as letter, but maybe that's the wrong assumption?

cc @fantasai @mstensho @AlaskanEmily

[mstensho]

I think it would make sense to keep the orientation that you'd get if no size were specified at all, i.e. it should still be UA-chosen.

Assuming that letter portrait is the same as letter would then be wrong. For WPT in particular, the default page size is 5in by 3in, so it's actually landscape by default.

[fantasai]

I suspect authors have some expectations when they specify one of these named values, and we should probably match them rather than leaving up to the UA... probably whatever the PDF renderers are doing, it's what we should be specifying. CC @MurakamiShinyu @faceless2

[MurakamiShinyu]

The spec says:

landscape …… The longer sides of the page box are horizontal.…
portrait …… The shorter sides of the page box are horizontal.…

letter …… 8.5 inches wide and 11 inches high
legal …… 8.5 inches wide by 14 inches high.
ledger …… 11 inches wide by 17 inches high.

I thought that "wide" means horizontal length, and letter, legal and ledger have shorter horizontal sizes, so these are "portrait" naturally (letter portrait = letter) and specifying landscape changes it.