In border-box mode, the computed height and width attributes give the border-box dimensions. This is contrary to Firefox which returns the content-box dimensions. I leave it to you to decide which is correct.
Created attachment 17103 [details]
The test case shows the width and height properties of the computed style. You can also check this with the Web Inspector.
An interesting detail is that the Web Inspector gets confused: If you inspect the square DIV in the test case and look at the "Metrics" pane, it will show the box as if its contents were 50x50, excluding borders and padding. This is incorrect because the border-box is in fact 50x50 (as it should be).
Created attachment 17104 [details]
Result with WebKit r27433
Created attachment 17105 [details]
Result with Firefox 188.8.131.52
Opera behaves identically as Webkit.
I *think* WebKit gets this right, and Firefox wrong, as by using getComputedStyle you're dumping the CSS-specified values directly and not a derived value. However I'm not sufficiently sure to close this bug, so I'll email www-style to ask for an answer as to which is the correct behaviour.
Regardless of the answer to this, the inspector's behavior is clearly incorrect and should have a bug filed against it.
Firefox is wrong. When "box-sizing:border-box" is set, the 'width' and 'height' properties control the size of the border box, not the content box. Thus, our behavior (of returning the size of the border box) is correct.