It also occasionally occurs with the mouse press/release also, but not to the extent of the keyboard. Perhaps we can rephrase the title of the issue as an HTML5 issue instead of a game related issue to up Google's "priority" a bit more?
I'm trying to check right now with some other HTML5 devs to see if they encounter the same issue with their games. Also, i've noticed in that Onslaught Arena game, they've somehow disable the whole entire web page except for the canvas area, thus allowing for any clicking(or accidental highlighting) outside the game area itself. Perhaps that can have something to do it with too?