Tiles show pixels from neighboring tiles when moving

[p-himik]

For example, open the Super Koalio game.
You'll see clouds like this one (don't pay attention to the top and right semitransparent borders - those are in the tileset itself, neighboring tiles are "leaking" a bit):

If you move a bit horizontally, you'll see some clouds shown like this:

Notice the green border on the right - it comes from the green ground tile in the tileset.
It keep flickering when the character moves.

And if you resize the window vertically, you can see clouds like this:

Notice the brown border at the top - again, it comes from the tileset, from the tiles above the cloud.

It works the same in both Chrome and Firefox, so I suspect the cause is somewhere in the code.