@callionica I believe the light DOM should be under the author's control, and the serialisation should represent changes the author made, not changes the browser made to itself.
This helps a lot with frameworks where you're trying to express "put the DOM in this shape" without overwriting interactivity, except in cases where you explicitly want that.
@kleinfreund I see where you're coming from, but you can also explain that by it getting the defaultValue. Your explanation might be more accurate according to the spec, but it duplicates some logic
If you want an element to fully contain margins (and floats, remember them?), but you don't want the contents to be clipped, display: flow-root is what you're looking for. https://jsbin.com/hafawud/edit?html,css,output