User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/38.0.2125.101 Safari/537.36 Steps to reproduce: In a hidden iframe, the computed style is not available, which is required in some cases. You can see this gist as an example: https://gist.github.com/skalnik/2f69478b435502357c81 When looking at `iframe.html` directly, you can see an output of "600px". When looking at it through `index.html`, you get an error. Actual results: `style` is undefined. Expected results: `style` should be defined.

In both Safari & Chrome the CSS styles are available, so the console.log output of "600px" comes through in `index.html`.

This seems to be related to bug 684197. From bz's comment there: > We don't perform layout in display:none iframes, because there is no sane > way to do it: a display:none iframe has no dimensions, but the sizes of > things inside it depend on the dimensions of the iframe.

See also bug 1011330 comment 3 for a possible solution ("we could just treat display:none as sized 0x0").

> We don't perform layout in display:none iframes, because there is no sane > way to do it: a display:none iframe has no dimensions, but the sizes of > things inside it depend on the dimensions of the iframe. That makes some sense, but I figured CSS attributes that aren't effected by the dimensions should still be computed, like in my example with `max-width`.

Styles depend on media queries, which depend on the dimensions. The right thing to do long-term is probably to treat those as 0x0, I guess. Insofar as there is any "right" behavior in a case like that: the computed styles will be different once the iframe is shown.