Trying to tweak a d3 image but don't want to fire up a full blown IDE? Give codepen a try. This nifty little tool bills itself as a "Front End Designer Playground" and auto updates, so you can instantly see how that tweak to HTML, JS, or CSS is changing your site. It's also super duper newbie friendly and free for the basic stuff.
And if you find that you can't live without it? There's a pro mode that supports collaboration, private projects, multidevice auto view, teacher mode, and uploading files among other things.