The default tab index of object should be -1

[rniwa]

#4759 made object element's default tab index 0 on the basis that both Firefox & Chrome do this.

However, this isn't true. Only Chrome uses the default tab index of 0 on object elements. Both WebKit and Firefox use the default tab index of -1 on object elements.

Given this, we'd like to keep WebKit's current behavior of not having a special case for object elements.

[rniwa]

@rakina @domenic @annevk

[domenic]

In https://boom-bath.glitch.me/tabindex.html the object columns are 0 in both Chrome and Safari.

[rniwa]

@domenic : That's because the object element in the document and therefore considered as focusable. That's akin to how an element ends up getting tabIndex of 0 if it's an editing host because it's considered focusable in WebKit. A better test of the default tab index is to create an element and don't insert it into anywhere in the document and check the value of tabIndex as in: document.createElement('object').tabIndex. This expression would return -1 in WebKit and Firefox and 0 in Chrome.

Since the goal of #1786 is to make tabIndex IDL attribute reflect tabindex content attribute, we shouldn't be taking such an effect (whether an element is focusable or not) into account when computing the value of tabIndex IDL attribute.

[domenic]

I agree we shouldn't be taking such things into effect.

However, when determining what the default is, I'm not sure whether we should be governed by the out-of-document case, or the in-document case. It seems like web developers are more likely to encounter the in-document case, so if we were to align with "the majority of behaviors web developers encounter" during our simplification, 0 seems like a better choice.

[rniwa]

Fair enough. Also see #4855