I'm drawing a map on a canvas using squares 40px * 40px.
All is well until I, by offsetting the canvas (using transform), scroll the map. Then, out of nowhere, lines apear between the tiles. See images below.
This looks like floating-point positioning (e.g. you've scrolled to 100.5, 100.5) combined with bilinear filtering most browsers use to display images on the 2D canvas.
Basically, if you drawImage() an image between pixels, every pixel is smoothed over two pixels, which means the edges draw at 50% alpha over the background. This breaks tiling, because the next tile's edge is the same, and draws at 50% alpha over the other tile's 50% alpha, which adds up to 75% alpha. This will appear lighter or darker (depending on the background color) than the rest of the tile. So you get "seams" along the edges of the tiles.
To fix: Math.round() your image co-ordinates, as well as any calls to translate() (since translating by half a pixel has the same effect). This guarantees everything is drawn to a pixel-aligned grid and never seams.