One of my biggest gripes when wanting to make a game that isn't 720p or higher, but closer to retro resolutions, but still in 16:9, is finding a resolution that fits and works.
I was testing some dialogue stuff (mostly just menu graphics and positioning, to see how it looked visually) and had settled on 768x432, using this list I found: http://pacoup.com/2011/06/12/list-of-true-169-resolutions/ (better than calculating the resolution every time you do something).
However, when I run the project the image wasn't stretched to the edges and had huge black borders around it. I tried every scaling mode and switched the browser(I mostly use webbkit, but it performed the same in chrome). No dice.
So I load up an example and change the resolution to 1280x720 at first, same deal. Poke about a bit, and suddenly it works as it should. Switched it to my desired resolution of 768x432, and it worked as it should. I have no idea why this is.
Not submitting this in the bug section because I'm not convinced it's a bug.
My test file:
The example I tried:
EDIT: After some sleep I returned to the issue and took another look. I was using the browser object to request fullscreen and it doesn't seem like the request fullscreen action actually changes the scaling mode.