There are many situations where it's not as clear what you're looking at if the content is fully (or mostly) transparent. The transparency grid is a fairly standard and well understood way of clarifying both the bounds of the content and that that particular "area" of content is transparent. We have lots of navigation items throughout Web Inspector for turning on/off the global "show transparency grid", but there's no item for it in the Settings Tab.
Created attachment 380392 [details] Patch
Comment on attachment 380392 [details] Patch r=me
Comment on attachment 380392 [details] Patch Clearing flags on attachment: 380392 Committed r250814: <https://trac.webkit.org/changeset/250814>
All reviewed patches have been landed. Closing bug.
<rdar://problem/56062269>