>>zootbo+ot8
It's not trivial to display ASCII art as text in html. There's so many ways to mess it up that unless the site is dedicated to ASCII art, a simple image is really preferable.
>>tetris+xN8
Sure, they might not be pixel-perfect, but if you're doing ASCII art you aren't really aiming for pixel-perfectness anyways, then you'd chose a different medium. It's inherently a medium for the clients to render how they see fit.