If a zoom transition is in progress, using zoom.event, a mousewheel gesture will interrupt the transition and cause a zoomend event, which resets the center0 variable to null. Previously, the center0 variable was assigned in the mousewheel handler prior to the interruption. This fix assigns it after the interruption.
During a combined zoom/pan gesture, controlled via simultaneous use of the mousewheel and dragging, it is possible for the drag to finish before the use of the mousewheel finishes. The mousewheel gesture is deemed to be finished if 50ms passes with no mousewheel event. Although the "zoomend" event is only dispatched when *both* gestures have finished, the variable "center0" was being reset when *either* gesture was finished, causing an error in the mousewheel handler. Fixes #2205.
Making D3 the subject of the last sentence makes it stronger and clearer. I also moved the adverbial clause "combing..." to the beginning because it was confusing at the end. We can now ask: "How does D3 give me the full capabilities of modern browsers without tying me to a proprietary framework," and get a clear answer: "By emphasizing web standards and combining powerful visualization components and a data-driven approach to DOM manipulation"
Update README.md to include a serial comma to better resolve ambiguity between the three terms HTML, SVG, and CSS. [Wikipedia](http://en.wikipedia.org/wiki/Serial_comma) states that an *omission* of a comma before the last term can suggest a stronger connection between the last two items in a series.