Oh, by pre-rendering, I meant using DHTML, not images. Each tile is a div. On the div, you draw a bunch of absolutely-positioned divs (sprites) with transparent PNGs for backgrounds. The sprites themselves are layered, i.e. you'd draw a bunch of divs with background images for terrain, then you'd draw resources at a higher z-index, then you'd draw cities, then you'd draw units, etc.
The reason for pre-rendering DHTML-based tiles is because a.) you have to worry about download times for the sprite images, though hopefully you can pay this once and have them cached forever and b.) it takes a fair bit of CPU to create lots of DOM elements and move them to the correct positions. Less than blitting an image to canvas, but more than just changing the top and left of a container div.
The reason for pre-rendering DHTML-based tiles is because a.) you have to worry about download times for the sprite images, though hopefully you can pay this once and have them cached forever and b.) it takes a fair bit of CPU to create lots of DOM elements and move them to the correct positions. Less than blitting an image to canvas, but more than just changing the top and left of a container div.