[css-page3] [cssom] Descriptors and properties

[AlaskanEmily]

Currently CSSOM specifies how properties are exposed on on CSSStyleDeclarations. Right now, Chrome and WebKit include a size property, reflecting the size "property" as here: https://drafts.csswg.org/css-page/#page-size-prop which can also be set on the CSSStyleDeclarations that is the style property of a CSSPageRule. However size isn't really a property in this sense, it behaves as a descriptor and is not valid as a property on the style of an element. Firefox does not allow it to be set on CSSStyleDeclarations, and it is only settable on the style of a CSSPageRule using setProperty.
It makes some sense that size could be settable on the style value of a CSSPageRule, so perhaps the style property of CSSPageRule should be a new interface that inherits from CSSStyleDeclarations but adds size and possibly page-orientation.

[dholbert]

To make this concrete, here's a testcase:
https://bug1867106.bmoattachments.org/attachment.cgi?id=9365919

The relevant style rule there is @page { size: 3px }, and if you view the output in web console, the last bit of logged output tries to get the value by logging this expression:

document.styleSheets[0].rules[3].style.size

In Firefox, this expression evaluates to undefined for reasons that Emily discussed above, which we think is correct per current spec, since .size isn't available on the interface. But Chromium/WebKit have it there anyway, and they log 3px.

In Firefox, the following longer version with getPropertyValue does evaluate to the actual 3px value as-expected:

document.styleSheets[0].rules[3].style.getPropertyValue("size")

so I think that's what web developers are currently supposed to do, but the fact that .size works in other browsers (and feels less-clumsy) makes it tempting to make that officially work for this situation.

[cdoublev]

I think this has been resolved in #5649:

So, here's a proposal. [...] CSSPageProperties for @page rules, with only the page descriptors

Is there any reason why we can't go for [...] CSSPageDescriptors [...] ?

RESOLVED: Having different interfaces for declarations blocks that expose a different set of descriptors/properties
RESOLVED: Go with emilio naming and Sam's amendment

In Chrome, fontFaceRule.style.getPropertyValue('color') currently returns green after fontFaceRule.style.setProperty('color', 'green'), which seems undesirable.

Similarly, I think keyframeRule.style.setProperty('animation-duration', '1s') should do nothing, as in Chrome but not Firefox, like tryRule.style.setProperty('color', 'green') (not supported by any browser).

[emilio]

Yeah, dupe of #5649. Self-reminder for doing the spec changes.