[css-viewport] [css-zoom] Should zoom affect iframes?

[chrishtr]

Issue #9442 is about replaced element intrinsic dimensions, but what about zoom of iframe contents?

Right now, the CSS implementation in Blink at least does not affect iframes. I think we should have zoom affect at least same-origin iframes, since I predict developers would want that behavior for ease of styling. For cross-origin iframes, we could not do it if it's judged to be a privacy issue? (Or the developer could opt-in via something like #1771).

@emilio @zcorpan

[zcorpan]

It also doesn't zoom the child document in WebKit. (demo)

A stackoverflow search suggests this is something that does come up: https://stackoverflow.com/search?tab=relevance&q=zoom%20iframe&searchOn=3

e.g.

• https://stackoverflow.com/questions/166160/how-can-i-scale-the-content-of-an-iframe
• https://stackoverflow.com/questions/47954638/zooming-iframe-content
• https://stackoverflow.com/questions/15003545/zoom-iframe-object-without-cropping

Maybe there's some compat risk with double-zooming if sites use both zoom (for IE) and transforms (for other browsers)?

[chrishtr]

I propose that zoom be applied to all sub-frames (including cross-origin), and be observable to developers via an adjusted window.devicePixelRatio (e.g. an "inherited" zoom of 2 for a frame would double its window.devicePixelRatio). This aligns with how browser zoom and screen density work.

It is also analogous to how color-scheme propagates to sub-frames (including cross-origin ones).

[css-meeting-bot]

The CSS Working Group just discussed [css-viewport] [css-zoom] Should zoom affect iframes?, and agreed to the following:

• RESOLVED: iframes will be effected by zoom, device pixel ratio will reflect it

The full IRC log of that discussion

<frances_> chris h: issues are related
<khush> q+
<frances_> PROPOSAL: Create iframe to improve zoom and note that pixel width has changed in the same way that zooming imposes constraint on ancestor element
<astearns> ack khush
<frances_> khush: any concerns on backwards compatibility issues to change behavior?
<frances_> chris h: will change behavior on web based browsers because they do not change, research does not involve iframes, want to ship in chrome
<fantasai> s/Create iframe to improve zoom and note that pixel width has changed/Have iframes be affected by zoom by affecting devicePixelRatio
<frances_> Alan: did the research use css zoom?
<frances_> chris h: yes, looked at office 365, none use an iframe in zoom to recollection.
<frances_> florian: very reasonable, is there a side effect in shipping?
<florian> s/is there a side effect in shipping?/if there is a side effect, we'll probably discover it by shipping?
<frances_> chris h: this aligns with other issue in objects to embed and make the zoom factor larger
<frances_> Alan: any other comments on zoom and iframes?
<frances_> PROPOSAL: iframes will be effected by zoom, device pixel ratio will reflect it
<frances_> Alan: any objections?
<frances_> RESOLVED: iframes will be effected by zoom, device pixel ratio will reflect it