they are not computed yet when we are trying to estimate margin top values. try to survive with the non-computed values. <body><div style="float: left"></div><div><div style="margin-top: 10px">foobar</div></div> when the floating box is placed, we need to know its final vertical position. We estimate margin tops all the way to the formatting context root. To computer margin-top for the <body>, we actually have to know if that nested div's margin gets collapsed all the way up. To find it out we need to know padding and border values. Unfortunately at the time when the float is placed we don't yet have that information.
Created attachment 356127 [details] Patch
Committed r238735: <https://trac.webkit.org/changeset/238735>
<rdar://problem/46372648>