The most common question I'm always asking myself is: Why is this like this? For many properties the browser can tell you which CSS selector "won" a property such as "font-family".
But for something like a width, the actual formula being applied is often hard to puzzle out. The browser will tell you its result but now how it arrived at that result.
For example, the element with id "pagespace" on this page. It has width that works out to some px depending on your screen width. But why that width specifically? I can personally figure it out by going up the DOM tree to see that each element is the full width of its parent (and that's how display: table-row, table-row-group and table all work) until we get to the "hnmain" element which is 85% width (of its parent). And then its parent is the body which has 8px margin all around. So the formula is something like: (screen-width - 8px * 2) * 0.85. But that took me how long to figure out manually?
Show your work!