Great sneaky prototype! I liked that you didn't immediately restart when caught like most stealth games. The sneak meter is an interesting concept with a variance to the player error. Might be a construct thing but the arrow keys had me scrolling down in the browser, for some reason focusing on the webplayer doesn't disable that, probably not your fault.
It'd be interesting if there were cases where you had to push through the boxes, maybe if the spacebar is limited there'd be a dilemma of having to push through the boxes to sacrifice sneakiness for utility. Maybe a timer mode would make that more mandatory. Otherwise a substantial and interesting idea!