I'm aware the blurry graphics is annoying but the situation is tricky!
You can force blocky (point sampling) resizing instead of blurry (linear sampling) resizing, by setting 'Sampling' to 'Point' in project properties, as you've found. However, it only has an effect in WebGL mode. Internet Explorer doesn't support WebGL, and Safari supports it but has it disabled by default AFAIK. Also, even in Chrome and Firefox, sometimes the user's computer does not support WebGL. For these browsers and users it falls back to the ordinary Canvas 2D renderer.
The problem with the Canvas 2D renderer is there's no option to set the sampling mode. Browsers choose whichever they feel like! In Canvas 2D, sometimes it will use linear, and other times point. I've contacted browser makers to see if there's any way to force it, but the impression I got was it'd be fixed in a future version of CSS, which is probably a year away or more!
So not much can be done I'm afraid. The only perfectly reliable way to do it is resize your sprites beforehand.