I've stumbled upon this tool while I was researching HTML5 game creation and it looked cool so I thought I'd give it a try.
I made a (very) simple project so I could see how your system performs.
The simple application is like that, there's a single sprite that responds to:
S - Rotate 45 degrees clockwise
Tab - Rotate 45 degrees counter-clockwise (As I expected it wouldn't work nicely, so don't bother with this, it will switch control to the browser and do crazy things)
Enter - Reset rotation
Now, that TAB bug is reasonable, but what bothered me was this. I tried adding some very basic CSS and other content (Text and image) to see if it would interfere with the game (Nothing serious, take a look at the source) , and then I found quite a weird bug. Seems like that after you click anywhere on the screen, the game no longer responds to user input!
No matter how much i tweaked the CSS nothing solved it, so I randomly pressed the keys and found something even weirder! After you lose control, if you press a certain key combination, and then click on the canvas, the control is restored!
After losing control, do: Enter, S S S, Tab Tab, Enter and then click on the canvas. Control should be restored! (It works for me most of the times, at least...)
The game: http://eztax.zxq.net/bug.html
The capx source: http://eztax.zxq.net/bug.zip
I hope this bug report can help you! I'm looking forward for more stable versions of Construct 2!