Non-pixel values for viewport in fixed layout

[dauwhe]

We create many fixed-layout cookbooks which are not print replica. We would like these books to "occupy as much of the available application screen area as possible," and set the viewport size to 100vw by 100vh, but the spec doesn't allow for non-pixel values.

Encouraging more responsive fixed-layout content might be good for both usability and accessibility.

[iherman]

+1

[GarthConboy]

Should be too late for major new stuff in 3.1 -- but, not an insane idea! :-)

There is certainly content floating around now that uses "device-height" and "device-width" for viewport aspect ratio (though not currently spec-allowed), but as it's in production content, it may be widely supported. Though, "device width/height" is not as useful as really referring to the ViewPort dimensions as proposed above.

There are issues with this request that would need to be considered -- for example, in landscape, should the width really be half the width for two-up pages? Would this be controlled by the rendition:spread and the RS would need to tune appropriately? What in the "auto" case?

And, I'm not sure how well real content using this mode would respond to layout in both tall/thin and short/fat aspect ratios. Could really good results be achieved?

[iherman]

But... isn't the usage of vh & Co. already allowed in 3.1 based on our own decisions? We rely on CSS snapshot for the CSS facilities; that document also refers to CSS Values and Units Module Level 3 as one of the constituent specifications, and that document defined the usage of vh in the section "Viewport-percentage lengths".

I understand the questions of @GarthConboy, and they have to be addressed, but that is not a new request for 3.1. These features are already "in" imho...

[mattgarrish]

They're valid in style sheets, but not in the viewport meta tag.[1] The Apple documentation also allows 'device-height' and 'device-width', but otherwise we'd be breaking from its definition if we were to allow anything other than pixels. We probably want to be mindful of that, too.

[1] http://www.idpf.org/epub/31/spec/epub-contentdocs.html#sec-fxl-icb-html

[iherman]

Ah. I did not realize that.

I must admit it is confusing if the meta element essentially disallows using a perfectly fine CSS. It is a source for confusion...

[JayPanoz]

And, I'm not sure how well real content using this mode would respond to layout in both tall/thin and short/fat aspect ratios. Could really good results be achieved?

background-size: cover | contain (MDN) and object-fit: cover | contain (MDN) + background-position and object-position should probably be mandatory for full-bleed then.

As for font-size, well, there’s vw and vh.

Question is—at least this is a question which comes to my mind—, what would be the benefits of such an option versus a blank page with media-queries à la web? From experience and for the record, the latter is something publishers are asking for as soon as we’re starting to design digital-only projects…

[dauwhe]

Yeah, interestingly <meta name="viewport" content="width=900, height=600"/> is not part of any real spec. The closest we come is a non-normative section of CSS Device Adaptation Level 1, which is only a WD. It does appear there are plans to revive the spec with an actual definition (since the normative parts of the spec with @viewport have been abandoned.