window.innerWidth vs document.documentElement.clientWidth

Question

Regarding window.innerWidth and document.documentElement.clientWidth,

1. Webkit (Chrome / Safari) claims innerWidth is smaller than clientWidth.

2. Trident and Presto claim innerWidth is bigger than clientWidth.

3. Gecko claims innerWidth is the same size as clientWidth.

What is the correct behavior stated by W3C (or silimar "authority")?

Test Script (on JSFiddle) (on GoogleHost):

(Run the snippet in full page mode and un-maximize or "restore" the window. Observe debug_msg while dragging the edge of the window to resize it.)

Answer 1

According to the W3C specification (17 March 2016):

The innerWidth attribute must return the viewport width including the size of a rendered scroll bar (if any), or zero if there is no viewport.

...

The clientWidth attribute must run these steps:

1. If the element has no associated CSS layout box or if the CSS layout box is inline, return zero.
2. If the element is the root element and the element's document is not in quirks mode, or if the element is the HTML body element and the element's document is in quirks mode, return the viewport width excluding the size of a rendered scroll bar (if any).
3. Return the width of the padding edge excluding the width of any rendered scrollbar between the padding edge and the border edge, ignoring any transforms that apply to the element and its ancestors.