I discovered this while working on bug 127447, and that's the only place I can reproduce it consistently. Attaching a reduced test case that should display the error but doesn't so there's an additional factor in the cascade or DOM that I'm missing.
Consistent repro is to:
1. Wait for bug 127447 to land in TOT.
2. Add "table-layout: fixed;" to the ".details-section > .content" selector in Source/WebInspectorUI/UserInterface/DetailsSection.css
3. Display the accessibility node inspector and toggle back and forth between an element with accessibility info (e.g. img) and one with no accessibility information (eg. head or style)
Created attachment 223298 [details]
incomplete test case
Created attachment 223299 [details]
screen shot showing layout bug in web inspector
Note the defined/calculated width in the web inspector is 85px, but the rendered width is 139px. This bug is resolved once you remove "table-layout: fixed;"